It can be super awkward. I've run into T a few times now. Once at the park when I was walking, and she was with her partner and her dog. Another time we ended up face to face in the doctor's surgery (I guess we must use the same group practise). Another time she and her family walked past me at a local market. And the most recent time I was at a music festival and I turned around to find her standing awkwardly behind me.
Apart from the time at the Drs surgery, where we really couldn't avoid each other, I looked away and didn't acknowledge her. I hate seeing her out of the the office..