Prius V losing antifreeze at higher speed only

Head gasket if there is not an obvious leak on the ground

 

What’s the miles?

 

Thermo itself...no.
Leak... yes.

I understand you are indicating that engine heat (higher rpms) expands "something" causing a loss of fluid.
Yes, knowing the mileage could be helpful.
Yes, as RJ mentioned, no coolant on the ground or elsewhere in the engine bay, could very well lead to a head gasket.

FWIW First thing I would do? I'd pressure test the coolant system.
(PS One item sometimes overlooked is the radiator cap itself leaking and evaporation being a loss of coolant cause.)

Good luck.

 

There might not be a radiator cap, assuming it’s like 3rd gen hatchback.