In my document I used images which are now gone (files were deleted) but yet they are still in my document. When packaging the document, InDesign says links are missing though. I don't understand this, if they're missing (I know they are) then how come they are displayed in the document? And what can I do if I don't remember where I found them initially?
I am not familiar with inDesign too much, but you could try the following in case you initially found the picture on the Internet...
Take a screenshot of the pictures in your document and use Google Reverse Image Search to find similar or the exact same picture. In case you got it from the Internet, you are most likely to find it doing so. Here is an article on how to use Google Reverse Image Search.
The images in your indesign document are a small copy of the images which are stored else where, the link is telling the document where it is, this keeps the indesign file small and quick to work with. But if you move the images or delete them, then the link is lost. My system is to store the image files and other input material, text or graphics in one file, named to the job so it can not be lost or deleted by mistake.
Another tip is that if the images have been scaled down in size, their 72dpi original resolution gets increased by the same percentage. For example, the original file might be missing but if the 72 dpi preview has been scaled down to 24%, the preview will actually become 300dpi and you can definitely get away with printing this.
To relink it, select the image with the content tool and copy. Then go to Photoshop and make a new document based on the clipboard contents. Paste it in, save the image and relink it to Indesign.
Alternatively, if you think you might still have the original images on your system but don't know where, try using the following AppleScript that can get Indesign to search for missing images through multiple subdirectories of a nominated folder: