Update: The issue is actually already known to Google, as explained at the bottom of this post.
Incognito mode usually keeps Web browsing history secret but search terms entered via Google.com are being retained, meaning that they show up as auto-prompts when searching from Google.com in regular mode. The glitch applies only to search terms made directly on Google.com, since items entered into the URL/search bar are not retained.
British company Parallax first highlighted this issue, which we were able to replicate as below:
Since it is such a specific issue, it seems unlikely to affect many users — but it is a concern since incognito mode is supposed to provide a watertight, anonymous browsing experience.
We’ve contacted Google for further details.
Update: It turns out that Google has previously noted the issue within its support pages, where it says the problem comes with developing for iOS:
On Chrome for iOS, due to platform limitation regular and incognito* tabs share HTML5 local storage, which is typically used by sites to store files on your device (client-side caching) or to provide offline functionality. This means the same sites can always access their data in this storage in both regular and incognito* tabs. Incognito* tabs will still keep browsing history and cookies separate from regular tabs, which are cleared once those tabs are closed.
Headline image via Roger Casas-Alatriste / Flickr