I'm starting to think this might be related to active content. I have 500+ listings and was using a template which contained JavaScript, so for the last few weeks I've been gradually updating them one by one with a new template.
The first "guidance" link started out with just a few listings but the number would get bigger every day. Tonight it was up to 180-odd, but I spent a little while updating several listings and now, for the first time, the number has dropped back down (currently 160).
When they introduced the new size requirements for images a couple of years ago, they did have a notification alert like that to tell you which listings had photos that didn't meet the new requirements (the number would change like this guidance one does, finding new listings every day, and dropping if you updated the images in the affected listings).
Still no idea about the second one that's appeared, though.
