2002 Ford Ka loosing water

When there's no obvious wetness on or under the engine the first suspect is a leaking cylinder head gasket. Your mechanic can do a chemical test at the radiator that may verify that, otherwise you can add a small bottle of dark purple dye to the coolant, then search with a black light after a few days. The dye will show up as a bright yellow stain that you can follow back to the source. If you see it inside the tail pipe the head gasket is leaking.

That is irrelevant. Cylinder head gaskets can leak coolant into the oil, into the combustion chamber, or outside the engine. They can also leak engine oil out of the engine or into the coolant, depending on whether the leak is at a passage with pressurized oil or an oil return passage to the crankcase. Coolant leaking into the combustion chamber, then going out the exhaust pipe is by far the most common failure. Coolant mixing with engine oil occurs in perhaps three to five percent of head gasket failures. The reason is there's only 15 pounds of pressure in the cooling system, about 40 pounds of pressure in the oiling system, but hundreds of pounds of pressure in the combustion chambers. That's where the weak spots will show up first, and the cooling system includes jackets around the cylinders. Being in such close proximity is why that is the most likely place for a leak to occur. The oil passages are usually pretty far removed from the cylinders and combustion chambers.