If this was a therapy clinic, I don't know why he wouldn't acknowledge you. I assume you weren't there with someone else. I would be pretty upset if I wasn't acknowledged at all. Hell, I saw my therapist in public once. He saw me but I hadn't noticed him until I saw him doing a big arm wave at me. He smiled, I smiled, we said hi and I went on my way. I remember this fondly because he seemed genuinely happy to see me out in public.

I imagine it must be really hard to see him in that setting though. I'd probably feel the same way.