Chet;
The likelyhood of one of those types having C-4 is pretty low. But, I'm a locksmith & sell safes for a living. It doesn't take explosives to open one up in pretty short order. I'm not going to put out on the internet how to do it, but I will say that it's not hard to pick up the info in a good reform school, county jail, or other institution of lower learning.
For the casual smash & grab thief, you're probably OK. If he's got 20 min & some knowledge, your stuff is gone. If there's two of them with a freight dolly & your Sentry isn't bolted down, wave goodbye.
One of your best methods is going to be to hide or disguise it.
900F