Coolant leak, stuck need help

I have been chasing this coolant leak and it is getting worse but it is random. It will not let out a drop for days then all of a sudden it pours out.

What has happened is the coolant leaks and empties like half the radiator. Has happened twice in the past week

What i have replaced, both recently and within the last year is
Radiator
All hoses to heater and radiator
heater tap
water pump
thermostat
radiator cap

The leak is nowhere near the front of the car nor the front of the engine block/pulley area. When it leaks it pools under the passenger side steering/passenger side of the sump area.
I know there are welch plugs, i can see 2 of them and they look good
A machanic has come and pressure tested the cooling system with no leak found. We let the car run for half hour and still no leak.
I took it for a drive, no leak.
Wife took it to the shops. comes out and of course it lost its coolant.

What is on the right hand or perhaps rear of the engine to leak ? it is in a spot where i cannot see it maybe under the exhaust headers or something like that
The oil is not milky, nor is the coolant contaminated.
Any ideas ?