I can just see the calm in the cabin on hearing an announcement that we're waiting for the Feds. The person under suspicion would surely remain calm.
I recently had cause to have the aircraft met by the Feds after two pax were suspected of removing crew name badges from stowed uniforms (no secure crew storage facilities). My PA asked pax to remain seated whilst we completed some formalities.
No panic. No alarm, and from the comments of the disembarking pax, they were in aggreeance with how the situation was handled. Advising that the Feds were on the way might have led to two guys who'd possibly over imbibed reacting in an unpleasant manner.