My R was doing this. It turned out that the problem was water was leaking out very slightly under pressure from where the water pipe goes into the block (I think this is the thermostat but I'm not sure). The part I'm talking about is on the front right hand side of the engine, where a large water pipe runs into the block.
The pipe itself can crack due to age, or develop a build up which breaks the seal of the clamp. I removed the hose, cleaned the deposits from the end and clamped it with a new clamp. This stopped the water leak.
The leak itself wasn't that visible as it was only a drip every 30 seconds or so. I could however with the benefit of hindsight see white coolant stains under the section that was leaking.
Hope this helps.
Paz