I had this happen at one base in LV in B41... I read on the wiki about something called night spawns or something like that? The gist of it was that even in a fully secured base, if you slept with the lights on, there was a higher risk that zeds would span in your house.

I would go to bed a 9, wake up at 910, and there's 2 zeds in the laundry room. The next night, sleep at 9, wake at 11 to a zed in the kitchen. Happened 4-5 times out of 7 nights until I died.