I can't say exactly when it started to leak - it must have been very slow as I didn't notice the drop in the coolant level (although I'm not that vigilant). The dealer eventually found the crack by pressurizing the system and using a black light - they showed it to me while the car was on the hoist. You could actually see the shape of the crack as the coolant oozed out, a hair's width about 1.5 inches long. I'm guessing it was a flaw in the original casting. I tried getting Volvo Canada involved using a "latent defect" argument and got a cold brush off. I had bought the car used a couple of months prior to the discovery - I suspect the vendor knew about it. I'll probably try to gouge out the crack with a die grinder and plug it with epoxy before going the welding route. It's been suggested to me to drill holes at each end of the crack to stop it from enlarging. I'm nervous about this approach as it involves first making the crack larger ! Thanx for the input.