I would suggest that you have a bad radiator cap or it is not seated properly. The overflow pipe from the radiator neck should also go into the expansion tank, not to ground?

It could be a defective cap leaking coolant through the overflow pipe into the expansion tank, which is in turn filling up past the high level mark and then leaking on to the ground through its cap (which is a bung and won't hold pressure).

A defective radiator cap won't allow the system to build pressure so as the motor warms up it will quickly discharge expanded coolant.

First check radiator cap. Then look elsewhere at pipework etc..

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