Surge Tank losing little Coolant !

check underneath and hose for moisture. also look at the parts beneath it and the ground. if it's slow enough, it might be evaporating

 

It probably is NOT evaporation, but might not be anything to worry about either.

Inspect the tank and connected hose for cracks and leaks.
If there is a leak, hopefully it will be there and not in the main radiator loop.

Then leave it alone until it gets down to the "min" mark when cold.
It might never go that low.

Also look at your oil dip stick and inside of the oil fill cap for an accumulation of white foam.
If present, that could mean a leaking head gasket.

 

The over flow is pretty meaningless on a G2 prius.

Take the black plastic cover off the top of the radiator to get to the radiator cap on the left. Dont be surprised if the coolant is low in the rad. It should be up to the neck.

Coolant should never go low if it does you have a leak. get a flashlight and look at the fornt of the engine where the belt is. Check the belt condition and look for red or green speckles in that area which is dried coolant coming from the blown engine water pump. Look carefully.
Coolant loss on a prius is concerning becasue there is no temp gauge on the dash you will never even know if the engine is overheating until it overheats bad and then it will damage the engine. Very bad design. many a blown engine on this site from that..

How many miles and have you ever changed the engine coolant?

lastly you must drive this car at least 10 miles a day if not your lifestyle of letting it sit will kill the hybrid battery. It is the #1 non stop post on this site. There all dying.

Don't drive the car hybrid battery will die. Garanteed. Your life will come to a real quick screeching halt.

 

And that last statement is just plain wrong.
Letting the car just sit will be hard on the 12 V battery but will not, in general, be at all bad for the hybrid HV battery unless it sits for a REALLY long time.......or if the HV battery was questionable going in.
HV batteries going bad BECAUSE OF NON-USE is not the top post in this site. Far from it.