Unfortunately, some people do things like this. A lot of fines are issued when a camera takes a picture of your plate. I had this happen to me. I did get it rectified but what a hassle. If you report it you are not likely to get your plate back but it may save you a fine. The "authorities" will consider you still responsible for all fines issued to your plate even if someone else is using it.
This past season someone stole my sewer tank discharge cap and the strap that held it to the trailer. A replacement cost me $10. I can't imagine stealing a USED sewer cap to save $10. Some people!