I’m not sure for how long, but Google web search is searching alt-text within unlinked images. Alt-attributes are serving as textual replacement within contexts where there are no images (it is not to be confused with the title-attribute, which serves as an image’s title, e.g. “Painting by Van Gogh”). Indexing this alt-text of course makes complete sense, but from what I often heard Google in the past would only search alt-text when the image is within a link.
If you want to see this in action, search for “google 2020” and you’ll find this page in the results – it does not anywhere contain the text “google 2020” except in the alt-text of an unlinked image, and the alt-text “[Google 2020]” is shown directly in the result snippet (if the keyword would have been merely “attached” to the page by someone using this text in a link pointing to it, this wouldn’t have happened).
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