As a fellow coolant-weeping-V6 owner I can offer my own experiences. I'm still in the middle of doing this myself, mine is/was producing a warm/burning coolant smell - seemed to be toward the scuttle, but no vapour visible. HBV seemed dry, then again sometimes a smell seemed to come form the front of the engine, too. Hoses seemed fine, nothing visible anywhere.
With plenum, fuel rail and sandwich plate off, a bolt jammed in one hose, and a schrader valve in the other, neighbor's bicycle pump attached and the system built upto about 15-17PSI I saw a weep from the coolant bridge. Saddening as this was only done a year or two ago, using brand new dowty washers. Of course what's been happening is it's leaking water - no smell... but only after a while then being burned/boiled off as it sits on the block. It also looks like the auxilliary coolant pump seal has gone, though I can't rule out 100% the hoses to it are past their best too.
I had confirmed she was not leaking
anywhere, simple process of elimination. Hoses=ok, HBV=ok, header tank=ok, cap=ok, My only other thought was the old oil cooler, however being under there not too long ago, again, this was discounted. After all, cant be the coolant bridge, as that's good as new!
Silly thought - if we are looking toward HG... could a cold pressure test as I did aid you? I'm wondering if you can build the system up to pressure, and assuming the system
is holding pressure, would having the plugs out help? if there's something delicate that could be prodded inside each combustion chamber as a 'dipstick' (plastic tie wrap?) - if that comes away with even a spec of coolant on it, you've proved your HG's gone. Obviously depending the severity of the leak you may be talking hours/even overnight.
It's a rather heath robinson/left-field idea, but you'll end up taking the plenum etc.. off anyway to do the HG, so nothing to lose, really.