Unusual find my iPhone alerts

So, recently the pass few weeks I have been getting these weird already from, “find my iPhone”. It is saying, “unknown device has been found”, but it will not show me the serial number or what type device it is. I have checked my car to see if someone might have put a Apple tag on my car, but did not see anything. It only appears when leaving my girlfriends house to come home. Has anyone had this happen to them?