[LeeH]

Thanks for all the suggestions. I'm leaning toward the damaged toilet theory.

Here's more info:
Dog? Yes, we have one, but we keep the lid down.
Leak? House is on a slab. No signs of a leak.
Siphon? Toilet in 2nd bath doesn't do this. Very close to subject toilet.
Partial clog? Maybe, but no signs of a slow flush.