Comment by cbzoiav

We're talking something the size of a cell phone that can be powered for days on a battery. Hide it near the cell tower and leave. It takes specialist equipment to locate it so by the time someone has figured out what has happened and either got that equipment in or just done a manual search it's been hours to several days.

Or for some mayhem just stick a few of them under seats on public busses / trains set to activate at the same time.