no windows were open without screens. I think he came through our cabin's ceiling. makes me wonder if there's a colony somewhere in the ceiling.
You should check the perimeter for any voids.
He could have also come in when you had your door open. Bringing in groceries or something like that. They can fly right in without you even noticing. And the fact that he is asleep during the daytime is a good thing. Means he might not have rabies.
I have heard of the Austin bridge. Up in wino country, we have a bat colony in the eaves; sometimes one gets into the hall closet. Two weeks ago, in 90 degree heat, I'm hearing his heaving and groaning sound as I make a sandwich. I ask the MIL "wtf" and she tells me it's too hot for the bats in their colony, and they're MOVING AROUND.