You can house rule it either way, but I think you couldn't trigger the alarm in the guard's tile. I'll clarify in the next rulebook.
It just so happens I was teaching this game to four people this past weekend, and that exact thing happened. The Juicer herself was about to be found out on her next turn - she was trapped in a dead end with the guard one space away and header toward her - so I suggested the alarm move, which I thought was a darn clever way out of a sticky situation, and allowed me to impress them with how the combination of game mechanics in BB presents so many opportunities for ingeniously escaping detection. Now I guess I'll have to backtrack and tell them we cheated a bit.
I have to say, though, they really enjoyed the game, and just barely won (playing the 2-floor intro version.) I have a feeling it will be requested again.