My AirTag start beeping when I come near it

My AirTag most of the time (but not always) starts beeping when I come near. I changed the battery, checked everything on Find My app and can't find out why.. If it is concerned that I am being "tracked", well it's my own AirTag... Any help why it beeps only sometimes? And also if I unlock my iPhone it stops. After a minute or two the beeping stops.