Sans is going to take advantage of his being a skeleton and spend a few loops blending in with them to see if he can get information. He spends three loops practicing to make sure he's got it down. If the skeletons are in use inside the Red Keep, he'll definitely check things out in there, but in general, he's going to try to spend some time around every area listening in. He does this for at least three loops--he does this for three loops because his secondary objective here is to check out and see if anyone is showing signs of remembering this loop without saying anything. Sans can do this in his canon without even retaining his own memory of loops, so I'm confident in saying he's extremely proficient in this kind of observation. He's not trying to specifically interact with anybody right now, so unless something really crazy happens with someone he's just here to listen in on info/observe.

Any DC is fine!