Air tag stopped working

HI

my cat now wears an airtag after he dissapeared for 4 days. I noticed this morning he is not wearing it and it hasnt updated its location since 00.38 last night. the battery was fine (it's only 4 months old) and another one he lost in the gardens behind my house ( i cant retrieve) still shows and is updating. seems an odd coincidence that he has lost it at the same time it stopped updating - almost like somone has removed it and taken the battery out (parranoid..??) any ideas whats happned here? thanks