I had a dog that had some dodgy lumps; bum, in his ear, gums, around his belly and on his chest. They sound the exct same as you described - hard, marbley shaped and not prticularly sore to touch. They were all benign cysts and most were removed without a problem. Some were left in because they were is hard to reach places and they weren't causing problems. Some dogs (as Karlin said wrt labs) are prone to these funny things but there is no need for major concern until it is confirmed to be something to be concerned about!
good luck at the vets