I was given a 2010 Indd file, but not the images that were linked to that file. I made the requested edits to the file and exported as a PDF. I noticed that the unlinked image files showed up just fine in the (screen version) of that PDF. I have not tried printing this PDF yet as I haven't been around a printer, but what is the point of having linked images if this exports just fine?
Two possibilities. 1) The images are embedded. 2) They’re low res screen previews but look okay on the screen. Much like a web image would but they’ll print horribly.
Look into the link panel this should tell you any information you need.