Losing Coolant

I have a 2019 Prius 90k miles. Lost heat one day and took it to mechanic, I didn't have any coolant. He filled it with coolant and heat came on. About a month later, the same thing happend. Had me take it back a week later and coolant level was low. Did pressure check, checked for codes nothing. Suggested I take it to the dealership. I did, they told me the same thing. Did pressure check, checked everything for leakage, everything is clean. No computer codes. No signs of problems with head gasket. I'm still losing coolant. Adding some in every week. I'm losing the coolant from the engine coolant reservoir.

Any help would be highly appreciated.

Maria