Tis quite simple really. If there's no water leaking out from any of the pipework and your radiator isn't oozing somewhere, it's got to be going into your engine and it could be doing so a number of ways:
1) Failed head gasket
2) Cracked head
3) Cracked block
As Redhand said, if your oil level isn't creeping up, the water is obviously being spilled straight into a cylinder and then pushed back down the exhaust. If this is the case, it will have bent a con rod without a doubt because water does not compress! (as I'm sure you know).
My engine keeps loosing its power and I'm in the process of tracing it down to being a couple of things. First of all, the fuel pickup pipe in my fuel tank does not have the steel gause / filter mesh covering the tip (as the older one used to have as standard) which is allowing all the crap at the bottom of my fuel tank to be sucked up straight into my fuel filter which is clogging once every two weeks. These bits of dirt are also getting stuck in the lift pump mechanism which is being jammed - preventing it from being able to pump any fuel into my filter housing. It was literally pumping nothing! It can be cleaned out however which I've done recently. Also, my front brakes have decided to start binding which kills the engine completely, especially on hills.
-Pos
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iv never heard of losing water through the exhause though
