I'm pretty sure they're watching you right now... all of them! Haha, what you say is true, and I'm sure it happens sometimes. But not very often, and not very likely. There are also a ton of privacy settings on GMATclub that allow you to disclose as much or as little info as you want... so I agree that there are def people that could use a little more descretion when posting... but that the same time, you probably aren't running much of a risk at all. Plus, Adcoms understand you're human! Just because you are posting on a forum, and talking with others that are going through the same process, doesn't mean they would look negatively upon it...
Nooowww, I have seen cases where persons have disclosed their company name, position, name, or title, and that companies IT program tracked them down IMMEDIATELY and told them to remove it. Haha, a nice little slap on the wrist.
So if i must make a recommendation to people, don't put your company name, don't put your name in the username or anywhere else, don't write your email address, or anything that would be easily identifiable to you. Other than that, you are good. In this day and age, I wouldn't be surprised though if it happens sometimes though, I continuely am surprise with what people disclose on the site...