Glad you posted this now as I have a similar leak which I can't find as well. I too thought it might be frost plug, but now I think it may be coming from the PS port for the water pump. The leak is non-pressure but very slow.

I tried to remove the power steering pump bracket to get a better look but after removing every bolt I could fine it was still stuck on. I'm going to do a TB service so I can strip everything and see what I can see.

Is it possible it's from a bolt that requires thread sealant as the bolt goes into the water jacket?

92 Montero LS 3.0L V6 Auto, Stock, Original owner, 185,800K miles