Fern hasn't quite come back to life yet, and Stan knows that's probably the case (Dipper just died, after all. He's well aware of how long it can take to revive). Still, he winds up swinging by Disaster House anyway, one hundred percent just to check on Fern and no one else.

Obviously winding up at Varian's door is a total accident. Pure coincidence. There's some perfectly logical explanation for it other than that he wanted to check up on Varian too. Probably.

So, he knocks on Varian's door.

"Hey, uh." Stan coughs into his hand, as though that will make this less awkward. "...You home in there?"