Fraud & invalid traffic · also called Pixel stuffing, Invisible ads
Hidden ads
Hidden ads are ads loaded where no person can see them, for example in a one-pixel frame or behind other content, yet still counted and sometimes clicked by software.
An ad can be technically "served" without ever being visible. Fraudsters squeeze it into a one-by-one pixel frame (pixel stuffing), place it off-screen, make it transparent or bury it under other elements. Google describes this as ad hiding in its list of invalid activity.
In a search-feed setting the equivalent is loading a results page or related-search unit invisibly in the background of another page, so that ad requests and even clicks are generated without the visitor knowing. The visitor believes they are reading an article or watching a video; a hidden frame elsewhere on the page is running searches.
Warning signs: ad requests from pages where the feed is not supposed to exist, sessions with searches but no visible lander view, clicks with no pointer or touch activity, and frames with zero size.
Feed code is not allowed to be framed or modified, so this is both fraud and a breach of AFS program policies. For honest buyers, the risk is purchasing traffic from sites that monetise this way. Post-click analysis and visibility checks are the usual defences.
Think of it like this
It is a newspaper printing an advert in white ink on white paper, then invoicing the advertiser because the advert was "in the paper".
Related terms
Ad stacking
Ad stacking is layering several ads on top of one another in one slot so that only the top one is visible while all of them are counted.
Invalid traffic (IVT)
Invalid traffic is any click, search or ad view that does not come from a real person with genuine interest, whether caused by bots, fraud or honest accident.
Ad request
An ad request is a single call from a page to the search feed asking for ads to show for a particular keyword and visitor.
AFS program policies
AFS program policies are Google's rules for anyone showing its search ads on their own site, covering where queries come from, how ads are placed and what traffic is acceptable.
Query fraud (fake search queries)
Query fraud is sending a search feed searches that no real person chose to make, in order to trigger valuable ads.
Sources: Google Ad Traffic Quality: invalid activity, Google AdSense Help: AdSense for Search (AFS) policies