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In the new house of blades book, it talks about using markers that are numbered for the bombs on the board from 1-6 and then hidden identifier what number is the real bomb.
But in the main rule book, you only get the number of real booby trap bombs purchased and are they are considered live until a trooper gets close, then roll to see what happens and could be a dud.

I like the fake-out with getting more than active bombs, this seems more plausible than hoping you get the roll you needed to set it off.