Since the FBI is a part of the Judicial branch and the US is run by checks and balances, it's either th Executive branch (the president) or the Legislative branch (Congress). My best guess would be Congress.
executive branch
It is illegal confinement when someone is being held captive in some way or has their movements seriously restricted against their will. This could be someone being held prisoner or a hostage.
Contact the branch where you got the car and tell them about it. It's likely that the item is being held in a "lost and found" cabinet at the branch. :)
Judicial Branch
The legislative branch has the right to impeach high ranking officials. Including Federal officers, military generals. But normally for someone to be impeached they must go through a trial, which would be held by the judicial branch. (Court)
it is held in the supreme court
A wake is held for someone who has DIED.
The executive branch can set a person free held by the FBI. The President can use his power of pardon and reprieve granted by the Constitution.
There is no time limit for being held against your will.
If the guinea pig was not held a lot when it was small, it will be unaccustomed to being restrained by someone's hand, and will be easily frightened.
Sounds like you are looking for "Habeas corpus".
It depends on why he/she is in jail in the first place. If the person is being held prior to trial, he/she will be released if he is found not guilty at trial. If the person is being held after trial, they will be released at the termination of their sentence.