I ran into my T in the convenience store, we actually worked at the same university at that time so I ran into her a couple of times and also my pdoc once. All I said was Hi and that was all they said too. The APA (the pdoc one) has rules about even disclosing that you are a patient but you can self-disclose if you want. So basically they are going to follow your lead but not really initiate anything for that reason, it would be hard if someone asked how you knew each other or something like that because they couldn't really answer, I mean if they didn't answer then everyone would know anyway. So next time go for it but keep in mind you might be self-disclosing just by having the conversation.