Ken, you said you leave food on the porch for the stray cats. Is it possible that some other kind of critter is opening the door wider to get to the food? Raccoons, for example, can open doors.
I leave the sliding door open just a bit for the ferals, but that's enough for the raccoons to get in too. They would not need to slide it open all the way. I don't usually leave food there at night because I don't want to attract raccoons, although I might sometimes if Ella and Bubba leave a bunch of it uneaten.