Ignore the daft ideas about surveillance! That's just teasing you.
If the intelligence or crime-investigation services really were watching him - which is highly unlikely - it would be because he might have been doing something you and I would be glad to know nowt about.
I used to use EP and vaguely recollect seeing the name, maybe even reading his contributions though I forget on what topics. Perhaps he never made an account on SW, or like me, he did but under a very different nickname and perhaps different interests.
Anyway, I'm sorry you no longer have any contact with him; but people do leave sites like this, or choose not to join them, for whatever reasons of their own.