In the last couple of weeks my Golf Plus TDi has developed a strange battery drain issue. If I leave the car unlocked overnight it starts fine the following morning. If I lock it at night it’s totally flat the following morning. 
Anyone else experienced this or know a likely fix?

TIA

Stuart 

I suggest a proper check of the electrical system to determine the current draw of various components to then isolate the suspected circuit to see if this resolves the issue