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Are you using any counter on your pages? There are some counters that can display popups on your sites (for example webstats4u.com).
Also the reason you might not see it is that it uses a cookie, to only display it once per unique visitor, so maybe it pop-ed up once, but you clicked it away not knowing it came from your site and since than you won't see it anymore since you have a cookie that it is already displayed to you.
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