These cars been over 50k when new sometimes had a tracker fitted when new. Even if the subscription expires the tracker keeps sending out its signal.
As they are fitted as a security item you won’t know were it is on the car .....this is going to cost you time but removing all the trim panels in the boot is a good place to start looking.
Look for none standard wiring around fuse boxes in boot, interior kick panels and engine bay 👍🏻
Member on the Lexus World forum had two trackers fitted, he found them both in the boot..each side.
The alarm has a battery back up inside and the battery can corrode with age...so also with checking out 👍🏻