This is rather mysterious and I don't know what to do about it. I have been placing small groups of files into my document and noticed that a few didn't show up by the pointer when I was placing them under their part numbers. Checking the links panel, I find that they claim to be in the document, but they are not indicated as being on a page, off a page or waiting to be placed. They are just no where. Note these are not multipage files that are not expanded. These are single page image pdfs.
Notice that 111 Rabbit Poster and 114 same sheet do not have the orange link to their location and cannot be found using "Go To Link". I cannot seem to delete these from the links list. I was also able to individually try again with those files and place them successfully. Can anyone tell me what is going on?
I would start by exporting to .idml and opening that, then resaving as a new .indd file and see if the phantom links have disappeared. Then place the files again.
Back saving to 17 seems to have cleared it up. Doing that was also fortunate as the original became corrupted and unrecoverable after inDesign crashed a short while later.