I am going to assume that your son in law had contact with either a lazy cop or an ignorant one. I say that as it is the only excuse for what he was told. With a vehicle tag a suspect in a trespassing incident has been identified and should then be contacted by law enforcement and advised not to return. Despite signs and fences, a person will have to be given official notice not to return before he can be convicted of trespass; at least here in Florida, except for specific areas such as schools and designated construction sites.
I am in that line of work.
Call the Sheriff again and get another deputy out there.
Do not grab a gun. Under no circumstances is trespressing a capital offense. Although it certainly makes you feel like killing someone.
Toby