Dark spots, HP5 120

Intermittently I see these spots on negs, visible in skies typically. They show up on the print and I can't spot them (at least with RC - too poor for fibre atm). I know there were some reports like this earlier in year with other Ilford films, but perhaps this is something else. Any hints appreciated.

HP5 120 in Patterson tank:
DD-X
Ilfostop
Rapid Fixer
Ilfotol for final rinse
Clean tap water throughout

I wonder if those are caused by air bells on the film, as they appear as light grey spots...