Recently cured a long term, and very annoying, water leak on my 2.6 It wasn't the HBV or the transfer pipe O-ring although I did replace these to rule them out. It did turn out to be the oil cooler cover plate sealant. Finally found it by stripping it down, thoroughly cleaning the suspect area and pressuring the system when cold, looked in the dark with a powerful torch and you could just see it slowly weeping. Before this I thought it was the power steering pump as the coolant ended up just under the AC compressor and evaporated leaving a slimy pink fluid just like ATF!
Decided to replace the oil cooler even though it looked in good condition (due to good maintenance i.e. keeping the coolant topped up and regularly replaced). Not too bad a job although getting to the bottom of the oil cooler pipes with a 19mm crows foot spanner was a pig! I also had to buy a full set just to get the 19mm I needed!
Happy days.