Your dog always does something because they are getting some pleasure out of it. Reinforcement for a dog does not always mean food. Some activities are reinforcing in itself such as digging, jumping, barking, and many more give pleasure to the dog. Dogs are extremely intelligent beings. No dog is doing any behaviour without a reason. Always try to understand what the dog is gaining out of that behaviour. For instance, if your dog is barking in the room alone, most of the time you will open the door - even if just to tell him to be quiet - and without realising we train them to bark if they want to see you again. This form of connection and attention is what provides motivation to the dog's mystery behaviours.