What is an alternative way to provide search-engines with data for indexing

I have a search term, and I googled that search term.

No relevant results appeared on the web search results.

I've flicked over to the image results and I have around 10 images from ads posted that I know are relevant.

I've checked the mark-up on these pages, and I've checked image metadata - but no sign of the search term.

Speaking conceptually for now, since I'd rather not share the search term, how can we feed search-engine crawl attempts with data associated to pages without including this data in the mark-up? A way of rephrasing this, I suppose, would be - from where else can search engines find data used for indexing other than mark-up?

Note

As well as view source I've also inspected the DOM as is when loaded (I appreciate Google is sophisticated enough to sometimes parse data resulting from async calls etc.).