First, I'll say that I would probably have done what you did in taking those items with me, but you really better check the laws in your state concerning finding dead wildlife. I'd hate to see a guy get busted for something like this when a phone call would have cleared it right up. During or before turkey season wouldn't be a good time to be found with a beard or spurs in the pickup without a tag to go with them.

I'd guess it was a predator of some sort...coyote or cat probably. I've found a couple myself and at least once there was no breast meat left like you described.