re: OT vets .. What's this on my dog?

Considering the breed, age, and occasional nose diving in the rug it is most likely what we refer to as chin furunculosis. Basically a really angry ingrown hair on the chin secondary to trauma and inflammation to the superficial layers of the skin surrounding the hair. Very common in boxers and other short or bristle coated breeds. The great dane and pitbull are other prime examples for this.

However (this is the disclaimer part), since I obviously can't see/examine/test it in person I'd say there is chance for benign growth and very low chance for cancer. As a fellow boxer owner/lover I keep a constant state of paranoia about them. If you don't know that boxers are predisposed to cancer, well, you do now (and should know anyway). Best have your vet do a quick check as you've already indicated.