Considering it's pretty rare to run into other people, he very well could be here and you just haven't caught him on the network yet!

[Unbeknownst to Aradia, if Papyrus was here, they'd know.]

Otherwise, there's always the possibility of him coming in with new arrivals, however that works. I honestly can't even keep track of time around here half the time, and I'm usually made of the stuff!