i found this on a rumors site, i'm inclined to believe it:
Let's just suppose you're one of those MBTA money truck guards - the ones that periodically empty coins and tokens into bags and haul them away in an armored truck. Let's suppose you carry a weapon - a .357 magnum - rather formidable, and very heavy.
On a particular day, let's say Wednesday April 2, 2003, you've completed your money changing at the Forest Hills MBTA station, and need a bathroom break. You saunter into the facility and, as filthy as it is, you really just have to go.
Let's say you've been trained in the proper handling of your large and formidable .357 magnum weapon. This training should, of course, have included the admonition that you should "never lose control of your weapon". This is vital as statistics suggest that most armed guards and police are shot by their own guns.
Let's just say that, despite this training, you remove your weapon and leave it on the filthy counter, while you seek relief. You should have been admonished to lock up the cash, lock up the weapon, and then do your duty, but today for some reason the training didn't take.
This is where things begin to go very wrong. Let's just say you really screw up. Completing your bathroom break, you turn to and saunter back to the money truck - oops - sans .357 magnum. It takes a few moments to realize that this heavy item is no longer rapping against your thigh - it is indeed, like your superb training, among the missing. Quickly returning to the filthy urination facility you discover to your shock - it's gone!
How does one do that?
We're not certain, but there's one thing we can say. The T Police are, sadly, the cause of most of their own misfortune.