If people are "outed" it's usually on the computer end. That is, a family member discovers where you've been by checking your posting history.
I know of one case where someone was "outed" by fellow (nasty) members of this board, after an online dustup. They sent evidence of the poster's activity to the elders of the hall. We've deleted the accounts of those nasties.
The elders at our old hall know of my activities on another discussion board, through an amazing set of coincidences. I actually ran in to a fellow congregation member on the discussion board. After a brief exchange of "twenty questions" we figured out who each other was. We never met in person, but our private confidences on the activities in the hall sure raised Cain. When the dust had settled, the elders had pitched a fit, my husband had switched congregations, and my fellow confidant had left the Witnesses. But I am still unsure if they had managed to connect that incident with my involvement here on JWD. They certainly gave no indication to me or the other poster that they knew about my other internet activities.
As a general rule of thumb, say only those things that those you love most know already. For children and partners of active JW's, regularly delete your cookies and your history. Password protect your computer. Never give out your password to anyone for any reason.