You said out the vent hose, right?
I put this in the same group as, 'my bike pees gas when it's on the side stand'.
Yep!
I used to put coolant in my reservoir tank every ride. After every ride, it would be low, and coolant evidently had come out the vent hose. I stopped putting coolant in...just checked the rad level.
The rad level didn't change in 1/2 dz. rides with the reservoir empty!
I don't see that you mention if your rad level is staying up or not. If it is, and the coolant is coming from the tank..don't worry about it.
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Sorry! Completely missed the '250' part.
<delete> <delete> <delete>!!!