Sans will introduce himself to Rainer as a teacher from the Bureau Academy. In an unprecedented and therefore worthy of note move, Sans is actually wearing an outfit befitting of a technomancer (specifically this super cool hoodie except in cloak form).

Sans is going to start out asking her about the skeletons. You can see why he would have an interest.

(Note for later, since this will eventually be relevant, Sans is recording this conversation on his bracer, which is covered by the sleeves of his cloak.)