I don't know of a search feature for highlighting form fields. However, it would be easy to create a PDF and open it in Adobe Acrobat or Reader. A colored bar will appear at the top of the document indicating there are form fields, and there is a button for highlighting them. Use this as a guide for finding the fields in your file.
Thanks for your response Steve.
I should have mentioned this too - I tried to use that feature in Acrobat, but it didn't highlight any fields in the entire document. This made me wonder whether you can have "hidden" fields, or whether you can use fields for some other purpose (i.e. something to do with buttons or other interactivity).
The only other thing I noticed in the InDesign document is that if I create a new button and then go to link it to a bookmarked location (destination), aside from all the bookmarks I've created e.g. "section 1", "section 2", etc., there are lots of other available destinations such as "Specific Content 1", "Specific Content 2", "Additional 1", "Additional 2", etc. These were also in the document when it was provided to me, but I don't know what they relate to or how to remove them. Not sure whether these might have anything to do with the fields that I can't find?
All very confusing...!
Further to the above - the destinations I was referring to above were all hyperlinks to text anchors. The weird thing was though that I couldn't actually find the text anchors that they were linked to, even though they could still be used as links to navigate to various pages. I think this might be an issue that when you delete the text anchor or hyperlink, the destination remains. You can clean up unwanted destinations through the hyperlinks palette with "Delete Unused Destinations".
Still can't find those pesky form fields though...!
I tried to use that feature in Acrobat, but it didn't highlight any fields in the entire document. This made me wonder whether you can have "hidden" fields
Form fields can be hidden. Selecting "Highlight Existing Fields" will not highlight a hidden form. In Acrobat, select Tools > Forms > Edit. This will bring up a list of all forms in document and the page of the form. By selecting form name in the list, the actual form in document will be selected, as opposed to just highlighted. This method will select hidden forms as well.
That's a good tip - thanks Jeffrey - that helped a lot!