How can I find the path to referenced photos?

The Find Referenced File command will not work if the file location changed, or even the disk name changed.

Best I can suggest:

In Photos copy the File name of one of those “missing” images from the Info pane.

With (some of) the external drives connected, paste the file name into Spotlight search.

Whatever the location of that file is, it’s also likely to contain more of the “missing” files.

Photos will only manage the image files for you, if you let it do it by copying the photos into the library package. When decide to manage the photos on your own and import them as referenced, it is up to you to keep track of them. You will be in for a lot of trouble whenever you need to move the files, if the drive is filing up or you need to replace it, if the drive starts failing, or you need to restore the files from a backup. A referenced library is a doisaster waiting to happen.

There are additional limitations:

• Photos cannot delete the original, when you delete the image from from the library. The referenced file will be left behind.
• You cannot use iCloud Photos with a referenced library. A. mixed library (only some referenced items) may cause Photos 5 on Catalina to get stuck on the icloud sync.
• Photos 5 on macOS 10.15.2 has a bug and cannot show referenced files on the map.

Sort of bizarre that Photos knows where it got the file from, but is unable to tell me.

One of the (many) reasons that makes Photos unsuitable for referenced mode.