Saturday Night Mystery

For this evening's adventure, hubby and I saw a bat flying around our living room while we were watching tv. Then the kitties saw the bat. One kitty caught it (fortunate, as it wasn't flying out the doors we'd opened for that purpose, and unfortunate, as bats are very good neighbors and I hate seeing bad things happen to them). Another kitty stole it from the first one. I stole it from the kitties (wrapped in a washcloth; never touch wild animals) and took it outside. I was debating whether it was alive enough to warrant calling a rescue, when hubby decided to poke it with a stick (such a boy thing to do) and it flew away into the darkness. The question is, how did the bat get inside the house?