If you saw something sinister in someone's posts, what would you do?
1. Write to that person privately about it, to see if there's an innocent explanation?
2. Report it to the administrators?
3. Start a discussion about safety precautions on the Internet, without saying or insinuating anything about that person in particular?
4. Post about your suspicions publicly, pointing at that person, in a thread where that person is posting?
5. Other?
Depending on the circumstances, I might do #1, #2, or #3, or some combination of them, but never #4.
ETA:
Something else I might try is to try to find some friendly questions I could ask the person about the suspicious behavior, searching for possible innocent explanations.