The hydraulic fluids I dealt with are usually mineral oil and stain like crazy. You would be able to tell if a container of hydraulic oil has water in it since they are immiscible and there would be a massive difference in color.Hydraulic fluid is usually an oil isn't it? But I don't think its the sort of oil you can steal and use for fuel or anything like that.
Chahuahui crew was talking about it last night and they reckon it's probably result of ground crew taking short cuts, like leaving the container that holds hydraulic oil open instead of sealing after each use that caused water to leak in.
1/3 water and 2/3 oil is a lot of water to be fair.
Water should never be used as hydraulic fluid, especially in an Alaskan winter. The expansion of water when it solidifies will damage the hydraulic system. I'm not excluding the possibility of deliberate sabotage.