Dark frame artefact?

[astrodog73]

I took some 10 min darks tonight as it was wet.... Atik 420 mono....
I was looking through them and noticed some unusual artefacts that donesn't look right -looks like dust/crud, but on a dark - certainly don't see this on lights, or other, shorter darks I've done...
Also looks to be a gradient?
Scope had its dust cap on, only light in obs was PC and a power supply display... ?light leak?
I'll certainly redo them in scrupulous darkness in case, but seems odd....
I've done other darks at the same temp but shorter duration, and never seen this at all....

Any ideas??