A. Hamster have little cheek pouches, where they store and carry food. Any chance his food has sunflower seeds in it? I bet that may be what you are seeing.
If you watch him when he eats, you may notice him tuck away a little snack for later-he didn't swallow it whole, its in his cheek.
If you keep noticing it though, I would take him to the vet. He should remove seeds but on occasion things can get stuck in there and start an infection.
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