Honestly, if he has given you no other reasons to be concerned other than that mark, then you are being too paranoid.

It could be a number of things. It could be a bug bite or perhaps something fell on him? There is no telling.

And for the record, other people might have had this happen but there have been times where I have found cuts and bruises on various parts of my body without knowing how they got there. Yes, this does include sensitive spots such as hands, feet, neckline, elbow, shoulder, and other places. It is not uncommon for one to cut or bruise themselves without them realizing what happened.

So yeah, if he hasn't given you any other possible hints that he may be involved with another woman, let it go. Otherwise, you might push him away and cause him to want to cheat for real.