Hypothesis: one of our two youngest cats brought it in from outside. Reasoning: they carry toy balls around the house. Sometimes I find toy balls out in the yard. The young cats bring in earthworms, mice, rats, and birds. Someone has brought in strips of stretchy window caulking, as well. Until the young cats learned to use the cat door, nobody brought in earthworms or caulking or any other inanimate objects.
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