unnamed faces photos

Imported successfully multiple iphoto libraries to Photos which then detected thousands of new faces on those photos which was great.

Was using the 'suggested faces' to clear them (individually and in multiple selections) and then they all suddenly went (I was pleased but thought it was a bit too easy).

Anyway it appears that the unnamed faces are still in the library but I cant search for them.

I can find them accidentally when looking at another photo and it shows the photo with the unnamed face that I can then name successfully.

Searching for 'unnamed' brings back no results.

Creating a smart album where Face is "unnamed" has no images either.

Creating a smart album where Face is " " doesnt give any results either.

Creating album that Face does not contain doesnt work as there are multiple faces in photos too.

I am running El Capitan but this was happening well before I upgraded.

Any ideas?