Whilst I said I wasn't going to give this any more oxygen, this issue of "tags" really bugged me. I felt my integrity was at stake.
Obviously they are used for SEO purposes etc, but I have never physically added any in the past.
So this is what I have found out:
1. 
Any member can add tags to any 
thread as long as they are logged in. It doesn't have to be a moderator or an admin.
To give you an example of this, go to the latest intro thread: 
http://www.dntrade.com.au/ophelie-flippa-here-t5841.html 
There are already two tags there (right down the bottom). On the r/h side there is an "edit tags" button. If you click on that you will see that 
anyone can add two tags. (Admins / mods can add more).
2. As a mod, I now know how to identify who places tags. By moving my cursor over the tags, it tells me who put them there (but not when). Unfortunately, I deleted the tags on Billy01's post before I knew this!
3. FPR very kindly spent some time yesterday having a look back at the database, and he found that these tags were placed on 27th September 2011. For privacy reasons (and because this matter doesn't need any further ventilation), I'm not going to disclose who placed them (so please don't ask me!) - other than it was a current member who has never been a mod or admin! If they want to admit to it, they can.
So mystery solved; problem solved (tags removed / and that original thread removed)  - therefore win / win.
Cheers, Ned