I think I would have done what you did. I hate sitting in the waiting room at my T office. I usually sit with my headphones on and avoid eye contact with the exiting patients. I did on one occasion bump into a co-worker leaving the practice one day. We exchanged a brief surprise greeting and have never spoke about it since. I think if I was in your situation and I made eye contact with the other patient, I would just give a friendly smile or head nod. I don't think I would raise the issue with my T, unless she caught the exchange and asked about it.