The code only appeared in an ad for a while (it has been replaced by an ad for something else). Sorry I didn't post it earlier but there's no reason you can't still use it. Next time I see one I'll try to post while it's still showing. Meanwhile you should keep watching. That particular one appeared on the page linked to the email with the message board digest (with the list of most active topics of the day).
I too checked my account and it shows the coupon codes I've used (don't know if I can use the same code more than once) but that's all.