and make sure it hasn't cracked.

If this is the original radiator with the plastic headers, you may have a developing or completely cracked neck on the top of the radiator that is being held together by the hose clamp but leaks under pressure of running and with temperature. Loosen the clamp and attempt to pull off the hose. If the neck comes off, you'll know (and it won't come off if the neck is sound.).

This happened to me - I found out by pulling on the upper hose and the radiator neck came off. The plastic rots over time and fatigues with movement. Many others have posted similar experiences. What is perverse is what you are experiencing - no obvious sign when the car is turned off, and the coolant often evaporates from the engine heat while you're driving.

You are lucky if you find this is the case and can replace the radiator which is not too expensive and relatively easy to do. As an interim fix, there is usually enough of a stub left to clamp the hose on to until you can get home. It doesn't sound like coolant is leaking through the gasket, and the M103 block is pretty robust so if the engine hasn't been abused, it should be fine.

You are trying to diagnose the leak by looking where it runs-off to the ground when the source is likely higher up.

Attached is picture of my old radiator (original from 1990 300SE)

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