When the HG went on my ST24, I only noticed because I was under the car looking at the state of the manifold studs in preparation for switching them over to headers.

On the rear bank near to cylinder 3 I saw a trail of oil coming down the block which was definitely coming from the join between the head and the block (not high enough for the manifold). I checked the schematics and did a bit of research, and sure enough there is an oilway and weak gasket point right there.

I took it for a 400 mile round trip without problems then decided I'd used all my luck up and fixed the HG myself, I figured it would be much easier to fit the new headers to the heads with them off the car (this was a VERY good idea now I've done it since there was one stud on each head that was proper rusted in and needed heat and a lot of room to turn it out which would have been near impossible with the heads on the car still). When I got the heads off the scorching to the head and damage to the gasket was evident right where I thought it would be.

I lost no water and no excess oil (though it was leaking, it was very slow and not noticeable), and power remained normal. I didn't bother compression testing it because I wasn't in denial, I knew the HG had gone