Many of these discussions state that in Spotlight "This Mac" refers to locally mounted volumes but that is no longer true. Mount disk images, even if on a boot or local hard drive, are not found in Spotlight searches. Likewise, these items aren't shared.
The only way to find Spotlight results for disk images is to start with that volume selected, start a Finder spotlight search, and the select that "folder" to seach in. If you have many large disk images, you'll have to search each one seperately or create a script to do so.
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