they feds would probably charge you for destroying their property.

(only half joking)

There was a famous case in the UK in the 70s

The security services bugged the HQ of the Communist Party of Great Britain (because the UK doesn't have a bill of rights it's legal to be a commie there). The members found the bug and destroyed it. They were then prosecuted (succesfully) for destroying government property - even though the government denied planting the bug, and in those days even denied the existence of the security services.

Their case was that the defendants believed it was government property and therefore their intent was to destroy government property.

So if you have a reason to be tracked by the police then it would be a crime to destroy it. But if you have no reason to believe you have done anything wrong then it's more reasonable to believe it's a terrorist bomb and you should call the FBI/DHS/Army etc

ah, the old "You Destroyed Not-My Property"

I can't imagine these things have "GPS RECEIVER" stamped on the outside. How are they going to prove you knew it was a GPS?

Given my experience in dealing with military intelligence types - they probably have "PROPERTY OF FBI - SECRET UNDERCOVER OPERATIONS DEPT" printed on them along with an address and an orange flashing light.

Plus, you don't call the bomb squad. You report a "suspicious-looking item" on your car to local law-enforcement.

If you think it was placed there by law enforcement and your intention is to cause maximum embarrassment- then you call the DHS/Fire/etc and assume that inter-agency cooperation is such that they wont have a note saying you are a surveillance target.

If you think that it's FBI/DHS device (ie if your skin color is between latte and espresso, or your name as an 'Al' at the front) then call local law.

Only if you have been stupid enough to disturb a potential car bomb to see whats inside.

Back when I worked for a defense company our cars were checked with mirrors on sticks everytime we went to work, we had special police numbers to call if we ever saw any suspicious activity around our cars or received a suspicious package at home.

Although they weren't very understanding when I was away on a trip and my wife opened a package containing a microcontroller I ordered - and started an alert!