1995 Volvo 960 4 dr w/SR (86K-original miles 2nd owner)

I have a slow coolant leak that I can't find (and neither can the Volvo mechanic). Any suggestions are much appreciated.

What has been done thus far:

1) new OEM reservoir tank.
2) new OEM hoses and clamps - upper, lower, and the 2 hoses supplying the oil cooler.
3) Technician took out plugs and peaked into chamber with e-scope. He informs that coolant would have cleaned "normal" deposits in the chamber.
4) Technician used e-scope and did not detect any leakage behind the dash. In support of his findings, I don't have wet carpets or sweet/antifreeze smell in the cabin.
5) I let the car sit overnight and pressurized the system. It appears to me that something is leaking near or behind the oil cooler or behind the harmonic balancer. The same technician that can't find the leak also installed a new timing belt, water pump, and tensioner. I took the front timing belt cover off but did not detect leakage out of the water pump. Is it possible that the water pump only leaks when the car is operating at relatively high rpm settings? The reason I ask is that I seem to lose more coolant the harder I drive. I am refilling the reservoir every 3-4 days. If the mechanic is mistaken I do in fact have a failed head gasket, would it be prudent to repair the current engine or order a new crate engine from Volvo? I saw one via Taka for $4500.