Is it even feasible to trust, as the issue suggests? We've probably seen movies or read stories about FBI or CIA officers, some computer-related tricks in a subterranean oval office, and some kind of red circle lines fixed on some person, car, or any other type of vehicle. It can also take the appearance of a dotted red light moving in a tick-tag pattern. Sometimes the victims who are being monitored have no concept of what is going on, as in they have no awareness their phones are being tracked, which begs the question of whether it is criminal to take such action without the owner's knowledge. Well, it depends since in some cases, swapping one's phone without his/her consent can really save the owner's life, while in others, it can do the opposite. So it all relies on why you're tracking in the first place.
Consider the following scenario: if someone is kidnapped and an ultimatum is issued, going the legal route to obtain some sort of license or document that allows the phone to be tracked may be a bad idea, whereas if you are in a position to perform such action, the wisest thing to do is to go ahead and track that phone before it becomes late. The only time it will be unlawful is when it is utilized for offensive purposes, which frequently leads to crime.
There are certain legal ramifications to illegally tracking a person's mobile phone. Some laws consider it a crime and impose a maximum penalty, while others impose a maximum fine. Although it varies depending on where you live, for US citizens, such behavior is punishable by 1 to 3 years in prison and is classified as a 'C' felony. However, if it stops or intercepts the communication, it is subject to harsher punishment since it is a 'violation of wiretapping law,' which is allegedly a federal offense punishable by a 20-year prison sentence.
In fact, there was a situation where a Dallas husband tracked his wife's phone without her knowledge and even recorded her phone conversations. When the wife found out, she didn't just lose her mind over the whole thing; she sued the husband for invasion of privacy.