No. The monitor does not belong to the OP even if the prior owner is a homeless deadbeat.
OP send the owner a certified letter stating that he has until X date - pick something about two weeks away - to come get the monitor or you will be sending it to animal control or to a reptile rescue. I would also start looking now for reptile rescues to see which ones are capable of taking it, not all can. Similarly not all animal control agencies are set up for reptiles, you may want to check yours to see if they can handle a monitor.