1) it was wired to explode, to cover tracks or harm people.
2) It was fired a long time ago, came through a roof/wall and landed behind a server rack and got buried behind cables. the hole in the wall/ceiling tile got patched and there it sat until someone moved the rack.Possibly with numerous people kicking it or seeing it and not thinking a second thought about what it might be.
3) someone came into possession of it and hid it there because it's a secure location with few people who come by. eventually either they forgot about it or they were not in a position to be able to retrieve it.
4) it's inert and he's pulling everyone's leg
I personally think 4 is most likely, followed by 3 and then 2 and then 1.
edit:
one more possibility, it's IS inert but that is undetermined, so the police err on the side of caution and proceed as if it were live.
edit the sequel:
the police don't know about it. I'm fairly certain this is all bullshit now: