School boy error I know, but I thought Muse used to mark an asset in the Assets panel that was being upsampled with a yellow triangle or something? I have an image somewhere in my site that needs to be enlarged but I'm sure in the past I could find it by looking in the Assets panel. In my example the Assets panel is set to show all errors at the top (I'm aware there's three pics missing before anyone tells me!)
This is hard to say. There are – that is may experience – some very rare situations, when this alert is appears to be incorrect. But normally it works as expected.
Out of interest:
If you can’t find the „culprit“, please reduce your site as much as possible by deleting elements and pages, and share this .muse file with us (-> Dropbox, CC Files,…) using these instructions: https://forums.adobe.com/docs/DOC-8652