If an IID BT is left plugged into the OBD port with the car locked & left for several days, is it likely / accepted this will drain the battery?

Yes. 2006MY 4.2 SC
BMW R1250 GSA Rallye TE
Triumph Tiger 1050 (sold)
1979 Pontiac Trans Am (sold)

I inadvertently left mine plugged in for about a week, it didn't seem to do anything, I know Gap says they can, I suppose it depends...!!! Pete
__________________________________________________
2014 L405 Autobiography SDV8 4.4 Loire Blue Ebony interior
2011 L322 Vogue SE 4.4 TDV8 Baltic Blue. Parchment over Navy Interior. Sold
2012 L322 Autobiography 5.0 Supercharged Ipanema Sand, Jet Interior. Sold
2002 L322 Vogue 4.4 V8 Epson Green, Ivory over Aspen Interior (Fatty Offroader) Sold
-Click for Project Fatty off roader-

I did the same Pete, inadvertently left it in for 3 days and went to drive it today and no response at all from the key fob.

It’s on the ctek now and after an hour or so, it responded to the key and locks / unlock ok - I’ll leave it on overnight.

So in my case (MY15 L405) I’d say leaving it connected does drain the battery - checking the IID manual it does say to always remove after use.

I spoke to Pat about it a while ago, he said they had problems with it sometimes draining batteries so they advise to disconnect, maybe it depends on what you were doing as it may get hung up connected to some ECU and just drain... Hopefully a good charge should sort it out...  Pete
__________________________________________________
2014 L405 Autobiography SDV8 4.4 Loire Blue Ebony interior
2011 L322 Vogue SE 4.4 TDV8 Baltic Blue. Parchment over Navy Interior. Sold
2012 L322 Autobiography 5.0 Supercharged Ipanema Sand, Jet Interior. Sold
2002 L322 Vogue 4.4 V8 Epson Green, Ivory over Aspen Interior (Fatty Offroader) Sold
-Click for Project Fatty off roader-