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This looks like bar rub - has your hammie been bar chewing. Even if you haven't seen them do it if there are bars in he cage or any sort of small hole they can push their nose in to chew they can get bar rub. You need to block off any areas they chew or better still move o a bin cage or glass tank they cannot chew on. Provide a large enough wheel (at least 8in) to distract them to use the wheel rather than chewing the bars.