As a vet, I hear this phrase all the time, whether from working/performance dog folks or from pet owners. It can be anything from dental pain to allergies to arthritis. Owners tend to think "well, they aren't really crying out in pain when they are doing their job and they're still eating, so it must not be that bad..." I want to caution people that just because they are not in agony, doesn't mean they are not in pain or that it's not causing a decrease in their quality of life or working performance. If you are walking with a limp, you are likely in pain. If you have a nerve exposed from a fractured tooth, you are likely in pain. If you have a subluxated hip from a malformed joint, you are likely in pain. You may be still able to function on some level, but it's not a great way to live.
We cannot ask a dog how much pain they are in, but we definitely know they suffer from pain. Pain also increases stress, which increases cortisol (a stress hormone) and can interfere with the immune system and decrease healing. Veterinary medicine has advanced significantly in its use of multi-modal pain management over the years and it doesn't need to be very expensive either. So if a dog has a bad tooth or is limping or is even itching excessively (which would drive many people crazy), get it checked out! If you don't feel your dog's pain is being addressed, be proactive with your vet and don't be afraid to ask if there's anything else that you can try. If you don't have a vet that is progressive with pain management, get a second opinion.