can't delete greyed out files on external drive

I've seen a few similar threads about greyed out files & have tried most of the solutions mentioned to one that seem similar to the issue I'm having. I'll try to explain as clearly as possible using one particular file as an example;

I had a large folder of files (50+gb) as a zip folder. This was added to an external drive whilst travelling. When I got round to unzipping the file & then putting the individual files onto the drive, I deleted the zip folder, which did seem to go to trash. I then emptied the trash. Some weeks later, whilst using the same drive I noticed it was running low on space so I searched files, ordered via size & noticed that the zip folder was still there, but greyed out & I can't delete it. On 'get info' it doesn't mention that it's an alias or anything like that.

If I search for the zip folder on the drive I also can't find it. It only shows up if, for example, I search 'all files' & then order via size.

That's one example, but there are also lots of other greyed out files where I've deleted old files or duplicates.

To clarify, this isn't an issue with trash not being emptied. The external drive has been checked in utilities also.

Any ideas what the issue is?

Any ideas?