What you have to remember, is that someone would need to really want to go looking in the first place.
Id be surprised if a therapist really would ever feel that need. Even if the conversation in therapy was 'I've written it all down on an Internet forum for PTSS, but I'm never ever telling you' - your story isn't of interest to them; why you aren't talking about it is more important.
But - let's say they were just nosy...and unethical...without knowing the user name, they'd be hard pressed to separate any of us from the crowd of PTSD sites and stories.
Same goes for colleagues, friends...and if someone was motivated enough to become a member and try and figure you out, they'd still have a lot of work ahead of them. It's beyond mere curiosity.
It's why we emphasize having a unique user name here, different from the other ways you may be known on the web. Also why we remind people about the member trauma diaries being exempt from search engines.
Having said all that, I personally went through many of the same thoughts when I first joined. It's really not uncommon, especially for PTSD sufferers. But the likelihood that anyone would have reason to try and figure out who you are, here, is pretty much nil. And then, their ability to isolate you from the crowd...very much like looking for a needle in a haystack.
Could it happen? Sure. But is it likely to happen - that's the more relevant question. In my opinion, no.