It sounds as though it is possibly the alternator.
Just because it charges perfect does not mean it is not faulty!!
The alternator contains a thing called a regulator rectifier, the reg part is what regulates the voltage supplied to the battery, the rectifier part converts the pos and neg pulses given off by alternator to just positive on which the car works.
This is done by rectifier diodes little electrical one way valves, these can leak, even when brand new, and so allow current to travel from the battery to the alternator so draing the battery when the engine is off.
To eliminate simply leave a few days with the alt or battery disconnected.
The problem is leaky diodes will rarely show up in tests, as alt still charges 100% perfect.
The earth strap will not cause a drain, but does need to be there anyway.
tabetha