I have found it is very easy for animals to learn to use pet doors. We have two neighborhood cats that just walk right in and make themselves at home.
One night when my DH was out of town, I heard a noise in the kitchen. I went in and saw a round furry animal that I thought was a dog, with his face in a bowl of dry cat food, chomping away.
I didn't want to scare it so I snuck into the bedroom and got my glasses, came back and discovered it was a HUGE raccoon.
It turned out it was a female, very pregnant raccoon who could barely fit through the cat door. She was just as afraid of me as I was of her and she had a hard time getting back out of the door.
After a month or so, she returned and started bring her baby raccoons in with her. It was cute until the first morning I stepped in a puddle of raccoon pee.
We would always know when they were in the kitchen because we could hear their nails clinking against the cat dishes as they washed the dry cat food to moisten it.
I learned later that raccoons can spread rabies through their saliva, and since they were sharing the water bowl with my ancient cat, that was the end of their nocturnal visits.
Before I took to locking the door, through, they pulled off some impressing feats, like dragg a whole bag of Science Diet almost all the way out the cat door. (When they couldn't complete their task they just ate the food out of the bag, like a feedbag!)
