Did you try saving it as xml, and search for <Link /> ?
This identifies an empty link tag.
Then you identify back that tag position in the .pdf file.
Hope it helps
I think what Régine meant to say was Save As XML and search for "alt="
You are looking for tags with an Alt attribute but no page content. This is not the easiest thing to find, even with this technique.
Dreamweaver is a big help with this because you can search for "all instances" of what you are looking for and then scan through the XML pretty quickly. Alt is an Attribute, not an element or tag so you can't search for "Alt=/ "
If you've opened up all the tags, be aware that the Tag with the Alt text must have content directly within it -- that is you cannot have content nested within another tag. If you have alt text on a tag that contains other tags that contain content Acrobat will consider the parent tag (i.e., the tag/element with alt text) as having no content. (I think this started happening with Acrobat X, but I could be wrong about that.)
Problems like this can drive remediators crazy. Another incredibly frustrating one -- finding an empty Figure tag that Acrobat thinks should have alt text. Ideally, there should be no empty tags in a document, but screen readers will typically skip over empty tags. Even if this does not meet ISO, an occasional empty tag will not sink the ship (IMHO), but it won't pass the Checker if it has alt text.
Question -- If the Acrobat Checker can find and identify Tags with Alt text but with No Content and Empty Figure Tags with No Alt text, why can't it pinpoint their location in the Tags Panel?? Is it because the programmers don't use the software they create? I don't know. I'd bet $100 they're not staying up all night trying to find Tags with no content! Which isn't to say they are not staying up all night doing something else unpleasant.
Tasks like this need to be eliminated from remediation efforts. There are tons of documents needing remediation, and looking for empty tags and empty alt text in instances like this is a monumental waste of time.