Water leaks and rev counters

I have a Stagweber header tank fitted to my Stag, which has a low-level switch that, when activated, sends both an audible and visible alarm to the driver. Earlier this year it warned of low water level every month or so upon start up, and a check confirmed that I had a gradual loss of water from the cooling system. However, I had a problem tracing the loss as there did not appear to be any drips under the car.

In the end I had a detailed examination of all the hoses inside the engine bay and finally saw that the hose from the LH bank of cylinders to the heater had a small crack