[WoodstockCat]

The GPS would also have to have some kind of radio, either active or passive so it could be detected, which would mean the terrorists could also detect it.

I thought about this also but it's obvious from the pictures of the orange jump suits that they are being searched so that means it's useless in shoes or clothing. The only thing I can think of is something you swallow that is bio-degradeable (may be reaching tin hat territory there)..

[RightWhale]

Something like an aircraft crash alarm or even a car alarm. It goes off, the good guys come running and maybe in time.

[Tree of Liberty]

sub-cutaneous implant