It looks pinched to me, like possibly he found a way between the lid and the bin and tried to squeeze through?
My snakes have crimps in their scales like that but on their sides were they bend to coil up. Also my snakes climb around alot and frequently bend scales, it looks just like your pics but on their sides and bellies. I just make sure their tank stays clean whith propper humidity so cale rot cant set in. Dont worry about it. However, I would double check the lid to make sure he wont try to get out again and succeed.
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