Sure looks like a slow, steady coolant leak into the oil to me.
But over time that would have resulted in a very definite coolant loss. None of this, "it might have lost a little but its hard to be sure" stuff with that. If that wasn't reported, then sabotage looks better and better.
I don't think plain water, like from dunking it in a stream or flood, would do that. There's something chemical going on there.