Mine did that a few years ago right after I took it out of storage. It was funny the gear possision indicator would show the next gear about the time I came to 4500 to 5000.
It was not causing any other issues with the bike so I did as sugessted at rode it about a week in warmer weather. It went away on its own after a few more chain cleanings. I would start with cleaning sensor after all it is easy to get to.