E46 330d coolant leak

[KM666]

So my 2000MY 330d dumped all of its coolant on the M4 last night, having inspected it this morning it looks as though the radiator drain plug somehow came loose and emptied itself. both of the blue screws are wet, the one closest to the front is the one that was loose so should it fail itd spray everything behind it with coolant. The exansion tank tooks bone dry extrenally and the pipes are all dry.

a) how likely is it that the plug somehow worked its way loose (the previous owner flushed the coolant so a possibility it was not correctly secured), and b) having a look over the cooling system in general I cant seem to find a coolant temp sensor anywhere.

I see loads of references to it on BMW forums as being located behind the driver side headlight on the hose that enters the radiator, mine hasnt got anything on it at all, just a hose going straight from the pump into the radiator. Now I can find tons of tecnical diagrams showing where the screw type sensor fits into the rear of the block but not a single picture of the 'other' coolant sensor location.

[Sim89]

Not beyond possibility that it could've come lose if it hasn't been tightened correctly.

As far as I am aware the only coolant temp sensor is on the rear of the block at the side.