Andrew Gabriel coughed up some electrons that declared:
Good point - especially if doing this in an office and you set the whole fire alarm system off.
You can get proper orange/red caps for the commercial detectors which I think is preferred as it's obvious to everyone that it's still on.
I'll add this with a side note about not capping off commercial detectors without consulting the local fire officer and not leaving any capping on when not required, in any location.
Cheers
Tim