I'm in object mode, there's a circle around the cursor - exactly like what would be there if circle select were active, but it's not active.
Single click on things does nothing, I need to double-click to select anything.
And hitting C simply enables the normal circle select.
I have no idea how to turn it off.
BTW - it only applies to this project, none other. So I think I just hit some key by mistake - Thanks.