by mistake i went to my site NOT logged, to the "my content" url and saw all my suscribed threads... i changed the 1 for another member ID and i saw his suscribed threads.... this should be private, right?
SITE.COM/index.php?app=core&module=search&do=user_activity&mid=1

yet i did not saw anything from private forums :(

That URL just looks like a member search to me, does not look any different to clicking to view your content. :)

i did a member search and the url was different, i juast had the filing this should be private :P

If the URL is anything like what's in your first post, that's just the user content search for that user, not their subscribed threads. The URL for your subscribed threads page does have have the member ID# in it, you can't fiddle with it to pull up someone else's subscribed threads list.