When the engine is hot the water will be near the cap, it falls
back to the fill leval mark when the engine is cold. If you have
plenty of water (up to the cap) it's unlikely that you are losing
much, either from a leaking radiator or into the cylinders.

If water gets into the cylinders, from a blown head gasket,
you usually get an appreciable amount of white smoke on
starting. Residue in the oil filler cap is often a symptom
of short school run type journeys, where the oil, and
engine, never get to operating temperature, for a reasonable
period of time. It can also be a sympton of a head gasket but
it needs to be a major gasket failure.