Ads disappear for 1 of 3 reasons:
1) Item is marked as SOLD - once an item is marked as SOLD it will disappear from the active listings in about 24-36 hours depending upon the maintenance cycle.
2) Ad Expires - When ads are listed you choose a length of time to run the ad for, once that length of time passes the ad will disappear from the active listings. However, if you know the direct link to the ad you can return to the ad and edit it to re-list it for another period of time if it has not sold.
3) Ad Deleted by Admin/Moderator - Sometimes we have to delete ads because they are dupes, messed up ads, spam, whatever. When we do so, we notify the person who placed the ad. Additionally, if in a prior version of the software the ad did not have an expiration date we delete them after a period of time so as not to have a bunch of stale ads, and again we notify the person who listed the item.
I suspect that if Dave didn't mark it as Sold as in example #1, that his listing time expired as depicted in item #2.
Thanks for your support!
Oh and the Answer to Dave's question is that Dave himself deleted the ad by setting a listing time frame which had subsequently expired, meaning the system did what he asked it to do when he listed the ad.
Ah Technology...