How do I keep my creepers from disappearing?

I spawned a bunch of creepers in the morning and locked them all inside a "creeper pen" I made. And then when night falls only 4 out of the 20+ in the pen were remaining. So where did they go? How could they get out when there was no exit in the pen for them? I don't understand??!!

Monsters can despawn if you get too far away.

• A mob will immediately despawn if ever there is no player within 128 blocks of it. Note that this appears to be a sphere, not a cylinder from map top to bottom.
• A mob that has had no player within 32 blocks of it for more than 30 seconds has a 1 in 800 chance of despawning on each game tick (1/20th of a second). That means that such a mob will survive for just under a minute, on average, from the time they spawned or were last within 32 blocks of a player.
• All hostile mobs in a world will despawn if the difficulty is set to peaceful, regardless as to where the player is positioned in the world.

Only hostile mobs, squid, and ocelots can despawn as of 1.2.3

This is REALLY late, I know. But use a name tag and they won't disappear. :) Hope you or someone else sees this and it helps them! I clicked on this link to hope to find the answer. I ended up doing it on accident!!! :)