Have a look at the incisors whether they are neat and even and not slanted, jagged or inwards pointing - that can indicate overgrowing premolars at the back; oral flush (fungal) may also be a consideration, swallowing problems, as well as messy drinking or rubbing the chin somewhere or through overgrooming or being barbered by a colleague trying to pick up the least bit if food etc.
Dental abscesses can cause pain/discomfort long before they become noticeable, as I know from my Hywel.
Sometimes you may never find out, but it worth giving it a try. My Llewelyn had a bald patch on his neck that was neither mites or fungal (he was treated for either to be on the safe side); sometimes the hair would grow back and then he seemed to be barbered down to the skin again... There was no lump and it didn't seem to bother or vex him in any way.