Yikes. That doesn't sound like fun.
I'm going to take the high road and not pre-judge the guy. I live in an apartment, and own an NFA item (a silencer), but it's just because I'm only going to live in Tucson for about two years, and it's cheaper and easier for me to rent at this time.
Maybe the guy was looking for a home, or didn't want to bother with home ownership at this time? Maybe homes weren't affordable there? I have no idea, but it's not my place to judge. Many apartment buildings have weight restrictions so as to avoid structural damage -- a 4,000lb safe would have been prudent, yes, but perhaps not an option in the apartment.
In the event that one's storing a machinegun in one's property, whether in an RSC or safe, an alarm and a little wireless camera set to transmit picture/video to a remote computer or email address sounds like a good idea.
Either way, three NFA items (M60, Uzi, and silencer) have been stolen, and two of them are irreplaceable machineguns. I hope that the guns are found, those responsible are charged and tried, and (assuming the guy is innocent until proven guilty, and the guns were "legitimately" stolen) that the guns are returned to their proper owner.