maybe it was caught on the fur and fell off there or perhaps your cat has a UTI and is avoiding the cat box due to pain

it is not normal for a cat to go outside the litterbox, so you should take your cat to the vet and let them check things out

if you cat has long hair or if he is prone to having hangers on due to a dietary change or hair in stool that could be an answer, as well

you know your cat, so if it is abnormal take your cat to see the vet or at a minimum keep a close eye on your cat to see what other behavior changes you are seeing (perhaps frequent trips to the litter box or squatting outside the litterbox)

if caused by urinary obstruction you do need to take the cat to the vet immediately, like the 24/7 emergency vet clinic