My auxiliary belt was overhanging the tensioner slightly and I was worried it’d eventually slip off and I’d lose power steering, charging and air con.
I’ve also heard that they can get dragged into the timing belt if you’re unlucky too.
For some time, mine was squeaking only when cold but after a cam belt and new aux belt it got worse and was squeaking all the time.
I just changed both myself rather than speak with the garage.