Birds, chipmunks, squirrels, voles, mice, rats, all of them will stash seeds for future use, and then forget to retrieve a lot of them. Birds are messy, too, and tend to get seeds stuck to them at feeders and later preen them out of their feathers.

A lot of people put melon seeds with the attached "goo" out of the bird feeder, I do it all the time, and the cardinals, chickadees, and many other birds love them.

Or, as you say, it could be a cross from the melon seeds in your compost. Hard to say where it came from

As far as what kind of melon, well, that's hard to say, too, and if it came from a grocery store melon seed, it is probably not the same as the hybrid parent anyway. When it's ripe, eat it, and hopefully it will be a good-tasting melon. And, don't forget to give the seeds back to the birds, maybe someone else will get lucky and grow an unplanned melon in 2011!

Add to the possible suspects deer. Deer will munch on melons and if one happened to then defecate in your garden, melon seeds may have been "planted"