Comment by benoau
I think it would be pretty obvious the devices were tampered with if you actually opened them, you're just looking for something inexplicable that is superficially attached to the inside of the device.
I wouldn't get hung up on what specifically they used because it could be eavesdropping or GPS tracker, a different type of explosive, a chemical etc.
Just adding to this for completeness sake that if one suspected a device of being tampered with in that manor I would not just open it. Maybe they did not add any anti-tampering switches or conductive strips but this would be the hard way to learn if they did.