howd they get in there?

Mobs spawn in the darkness, my cells are all dark but small, so mobs will spawn every now and then, but not at a high rate, this time, it was 2 creepers.

I'm using this method in another room, I seal it off with cobblestone and open it with a press of a button, when it's closed (sealed), it's 100% dark, so a lot of mobs will spawn, I have a switch that will put in some light when I pull it, so every 10 minutes or so, I go down, open it, light it up, and it has around 10-15 different mobs, so I start farming XP and mob resources.

EDIT : For example I just game down to my prison now, found 2 creepers in 2 different cells and a spider in another one.

thats cool

lol silly creepers, don't they know they can explode their way out of there. =P

Actually, you're the one who's silly, they can explode, but not their way out of there :P

Cobblestone walls. One explodes in the front corner, taking out the front wall and the one joining the two cells. The other one walks down the corridor to the staircase leading up to the surface, reaches the door, and boom. Out.

You still didn't get it, they can explode, but after that, they can't go out. Think about it.