I have a fairly standard Hotpoint vented electric tumble dryer, which normally works as you would expect, drying clothes, venting the humid air out the side wall, and with a lint trap I empty every load.

Over the past couple of months, though, I have seen a few shirts come out with tiny (1 to 2 mm) scorch marks and am a bit baffled as to the cause.

I can't find any marks inside the drum or door seal, and there doesn't appear to be anything that looks burnt in the lint trap.

I can't find this symptom described anywhere using a quick google, so does anyone know what could cause this or any area I should be looking at to try and diagnose the issue?