A search query typed in Yandex is spotted in the Webalta advertisement

Today, at one of the resources in the network, I noticed a banner on which the Webalta search engine was advertised. A search form with the query “reed flower” was drawn on the banner. I could not believe it. Today I have just looked for this phrase in Yandex. I do not use webbalt at all. How they found out that request and slipped the banner with him exactly to me, remains a mystery. The banner was shown through the AdRiver system. When you click on it, hit the search results for "reed flower" in Webalt. At the same time, the request was not transmitted in the link.

Contextual advertising turned out to be good, but it very much borders on the violation of the right to privacy. It was on the page with the banner there were no reeds, the transition to it was not carried out from the pages on which the reeds were mentioned. And since the request is very rare, I cannot agree that this is just a coincidence.

Added by:
As a result of the discussion and tests, a rough outline of the banner.
1) Previously, a transition was made from Yandex to the page where the AdRiver ad was installed.
2) AdRiver saved a search query in his database and assigned me a UID, which he saved in a cookie.
3) When I visit the pages with AdRiver ads, banners are created “for me” according to UID.