I sometimes do, but only to see if there is someone missing for a while then I'll PM to see if they are doing ok. If I don't see them on the forum or their name on the users list and they've had a bad time. then i will PM them. There is always invisible mode.
I don't see it as invasive for someone to check whos reading my threads etc.
People might see my activity as invasive but I like to make sure people are ok as many people disappear and their name turn up on the grief and loss forum, I think this very sad.

In short, no not being intrusive.