The fire service pretty universally switched to Knox boxes probably 30 years ago. These are controlled key boxes that allow the fire department to obtain access to the boxes. Your rank and file member doesn't have access to it, only the officers and most departments have rather strict controls on this. However, I can tell that 9 times out of 10, we don't even use them, we just break in. I've got a thing called a K-tool than can rip a lockset out of the door in short order. I also know a number of way to defeat most retail store doors (cheesy). There's always axes and other implements of destruction.
I believe CDW is right, most departments will likely not be able to ask about arrests that didn't lead to convictions. However, as pointed out, they have better avenues to background checks that might find them as well. They also may inquire with the local police and fire departments where you have lived to see if they have any experience with you. I can tell you for a fact when I was in the fire service we talked to every organization (employer, volunteer fire company, other career companies) that you were (or might have been) affiliated with. There are a lot of people ranging from firebugs to just those who go bananas with lights and sirens that have already been asked to leave other departments who are the last ones we want to hire.

