We've noticed this a few times lately with Mumble. We'll be inside, moderate temperature, not doing anything physically exerting, and he'll be panting. Normally I'd think maybe he was scared/nervous but everything about him looks relaxed and happy. Just now it happened while we were all just chilling on the couch, watching videos. Completely relaxed posture, mouth parted and panting.
A very quick google search suggests that sometimes dogs might pant when they're happy. I can't remember a dog doing this outside of exercise. Not that I think it's a problem I guess I'm just curious about other people's experiences.