Sans is a MONSTER and not a HUMAN, so he doesn't follow HUMAN biology like a HUMAN skeleton. Same with Gaster and Papyrus.  

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Sans made a mental note that he would have to thank his father the next time they spoke, considering it was all thanks to him they had even managed to make it from A - B in one piece. When leaving the headquarters, the trio had tried their best to blend into normal society, just walking along, trying to act as normal as they possibly could. It was clear that Undyne hadn't given up on finding them, because every now and then when they would walk down a new street, it was becoming old to see a police man or woman loitering around on a street. But as they passed by the cops, they were never picked out from a crowd, (who would be crazy enough to guess that two skeleton monsters had somehow gained themselves skin, eyes and hair?). 

It took at least two hours to get there on foot without any kind of travel, but that was fine. The three of them hadn't wanted to risk taking public travel in case someone who was looking for them did somehow make the connection and they got trapped on the bus with them. Plus all that Frisk had was a shorter hairstyle and some make-up to make her look as different as make-up would allow. If someone took long enough to properly stare at her, she could be recognised. So there was that. 

By the time they got to the location, it was starting to get dark, so just to be safe, the oldest of the group, (that being Sans), told his girlfriend and little brother to stay close to him, just in case. Right now, they were currently down a lonely dark street, the other side of town. The road that they were currently on was one that began to stretch out for miles, away from this town. Just ahead of them, they saw a lonely car parked, and it was difficult to tell if anyone was sitting inside. Well, at least until the person at the driver's seat decided to randomly turn on their headlights. 

The group of three squinted, trying to cover their eyes from the light, at least until it finally went out and two doors - one on either side of the front of the car began to open. To their surprise, a monster that had purple skin, five eyes, black hair tied up into two ribbons either side of her head. She also had six arms, and two legs - like that of a spider, and that's what made Frisk assume she was kind of spider monster. A spider monster wearing a black blazer on top of a white shirt, along with jet black suit pants. 

"I'm guessing you're who was waiting for us here?" Frisk questioned towards the monster, who nodded. 

"Yes, my dear, you would be correct. I am Muffet." She explained, before turning towards the other person who was standing in the shadows beside the car, so they hadn't really been able to see their face very well. "And this is my accomplice who is also here to help." She went on to explain, saying that as an invitation for the other unknown being to step forward to introduce themselves. Out of the trio, there was one person who recognised this person, and he was definitely full of shock when he saw them.

"M-M-METTATON?!" Papyrus cried in surprise, causing both Sans and Frisk to look at him. Doing a double take, Sans glanced back towards the robot and that's when it clicked - he remembered he had seen this same guy at that party so long ago now, before he and Frisk were even dating and he was so madly in love with her. This guy had wanted to dance with Papyrus, and he allowed them to. Mettaton - the robot who his little brother probably had a thing for. 

"Hiya, darling." Mettaton commented with a teasing wave, chuckling a little in that robotic voice of his at the look on Papyrus' face. It was a mix of confusion, surprise, happiness, and even perhaps a little bit of horror at the same time, (maybe because he was worried Mettaton was going to tell Undyne on them). "I can see you're confused. It's probably a good idea I explained." The robot grinned, placing a hand on his hip, while Sans crossed his arms with a risen eyebrow.

"weren't ya part of the CIA?" Sans questioned. "what the hell are ya doin' here?" He asked, feeling somewhat distrusting and suspicious of the robot, who glanced in his direction at his question. Mettaton appeared to laugh in an interested fashion, as he commented on how it was extremely interesting how he and Papyrus were humans at the moment. That it was a nice touch, but Sans just growled and told the guy who was already getting on his nerves to answer the question. Exhaling, Mettaton decided to answer.

"Pfft, you're no fun. But if you really insist on an answer, I'm sort of a double agent. Well, more like a spy." Mettaton explained. He never really was one to be of help to the CIA, he had just sort of always been there because of Alphys, and Undyne never questioned it.

"The entire time I knew the truth about you, Sans. Because I knew Grillby, thanks to Muffet. I knew that you were part of the Mafia, but I just played along, pretending I had no idea who you were, or that Papyrus even had a brother, just so I didn't give anything away. And because I knew it would be bad for you if Undyne were to find out what Papyrus knew about you, I purposely tried to intervene in the situation, trying to stop her from doing a lie detector on him. That and... I didn't think it was right, it was too stressful for him." Mettaton went on, flashing a weak smile towards the younger skeleton (human), while Papyrus felt touched that Mettaton had tried his best to protect him, and even his brother too.

"...i guess it checks out then." Sans sighed, deciding to back down with his threatening posture. "so, how is undyne doin' at the moment?" Sans questioned, while Mettaton's expression changed to a frown and he shook his head, going on to say how she was still pretty furious, and wanted to try her best to find them. How long it would take her to give up, he did not know the answer to that. But he would try his best to keep them out of heat when he was playing his role as a member of the CIA.

"Enough talking, dears. We must get a move on before someone sees us here and finds it suspicious that we are loitering. Get in the car." Muffet instructed, pointing the three towards the back seats of the car, and there were three - just enough luckily for them to sit in. Sans decided to be the first to climb in, while Frisk sat in the middle and Papyrus sat the other side. It was difficult not to notice the fact that his little brother was still smiling, likely because he was happy that Mettaton was here. Sans couldn't help but quietly chuckle at that, finding how Papyrus admired the guy strangely adorable, (of course if he hurt Papyrus, Sans would have to kick his fucking ass and give him a bad time).

"SO WHERE IS IT THAT WE ARE HEADING?" Papyrus decided to ask, as the car started up, roaring into life while Muffet sat in the drivers seat. Glancing towards Papyrus, Mettaton told him how they were going to a whole new town - but it might take awhile to get there, so they were probably going to want to sleep before they arrived. Frisk didn't mind such an invitation; neither did Papyrus. But Sans being his normal, finding-it-hard-to-trust-people self, he felt like he wouldn't be able to sleep in the car with two people he hardly knew (even if Papyrus knew one of them).

As the car began creeping forward, the oldest skeleton (human) found it difficult not to frown at what they were leaving behind. Family, friends, the apartment... Their whole lives, and it would probably never stop - them fleeing from place to place. One day they might get found out in this new destination they were heading to, and then they would have to run away once more. Looking over to Frisk, who had decided to rest her head on his shoulder, he went to softly press his lips against her forehead in a kiss, hoping that she would sleep well. Somehow, he would keep protecting her. 

They would start their new lives here in this new destination.