I would carefully go back & verify this, mark the tank level before & after. Make sure the sled is in the same level plan before & after. If it is true should be easy identify the culprit.
As far as your coolant levels I don't see any connection to the oil unless you have had to add. It's normal for the level to change as the system develops heat & pressure....release the pressure & the level will return to normal.
By chance have you checked to see if pressure is building in the oil tank?
Well said! Graduating the oil tank is also an excellent idea.
For the coolant level, if you have ever connected a vacuum/pressure gauge to the coolant tank vent with an unpressurized cap on a two stroke sled, I imagine that you have noticed it draws a vacuum as you depress the throttle. It is one of the rare subjects that have yet to surface on DooTalk other than what can be expected for the coolant level as the norm. Usually the only explanations given are those for automobiles which don't directly translate to our two strokes because of the difference in the pressure cap.