I worked for a PI company as a surveillance/pursuit driver. There has been some really bad advice offered in this thread, and some extremely prudent advice as well.
I can only speak for Massachusetts, but in that state PIs are licensed. They have no special legal powers and most of them do not carry while on duty. All of the street investigators at my former company were highly professional, patient, and cool-headed. If you get frustrated easily then PI work is not for you. I left the job because I became uncomfortable with what breaking all those traffic laws was doing to my auto insurance. Simply presenting my PI license to the cop that just pulled me over never worked. Oh, well, I guess I needed to work on my smile.
If you start screwing around with the pursuit team they will either,
A: Not care because they're being paid by the hour.
B: Realize you are screwing with them, and go tell the client the game is up.
If we were confronted, we would admit who we were but say nothing about the case. If the individual got upset we would simply leave. In the 25 years that this company was in operation they never had any major problems. Hotheads need not apply to this company, and those that did were never employed for very long.
My company was in the habit of informing the local Police Department that we would be operating in their town/city that day, if we knew we were going to be there for more than a few minutes. Therefore, most people who called the cops on us knew right away who we were.
Davidtdm, these guys sound like a bunch of amateurs. I would definitely take whatever legal recourse you can.