The first HUGE wave of negative trolls came in June 2011, so when you see "Hero Member" there is no guarantee they are here for positive reasons.
Surely they didn't stick around and make a post every fortnight to become a Hero member. It's rare that you see a Troll Hero member as well.
You can be completely away for over a year and still have made "Hero", but you are correct that many of them probably have lower activity levels.
You can be away but if you don't make a post every two weeks you wont magically become a Hero Member any sooner. It's not like you could be a noob, leave for a year, log back on and get all those activity points you missed. You still have to earn it. It's a good system.
And I can think of a few troll hero members, but none of them are really malicious, just annoying lol.