So my dad actually worked in a bank for a majority of my childhood and had two different robberies occur at his bank that used a similar method. Both robbers brought what they claimed to be explosive devices and said they would blow them up if they saw the cops...both times the alarm buttons were pressed. However, what you don't take into account is that at most banks the tellers are trained to just give the robber the money, it is insured for the bank, so giving it to the robber doesn't hurt them. In both robberies the robbers got away from the bank, but they didn't account for the ink dyes put in with the money they were given, which burned their skin making them easily identified, nor the fact that there were multiple people watching the outside of the bank to identify the get away car. Both robbers were caught within a day or two. It's not a bad idea, it just doesn't account for too many variables once you leave the bank, or the fact that pushing the alarm button isn't intended to trap the robber, but rather to have the cops on their way to the bank quicker so they can get on the robbers trail sooner.