2013 X3 - my wife's daily drive so I am not intimately familiar (with the car!). 

A strange problem has started occurring recently started maybe 4-6 weeks ago.

The tailgate is opening uncommanded - i.e. no switch or button pressed.  Sometimes when car is actually moving, which is disconcerting, to say the least.

Last time was yesterday after heavy rain on Saturday (not used Sunday).  It was okay when my wife left in the morning but at some point in the day, she reported that the tailgate opened uncommanded immediately after a firm braking event.  It happened to her again later in the day.  I picked the car up from her but could not replicate the fault.  I removed the tailgate inner panel to check for evidence of moisture in either the "close" button on bottom face of panel (operated to close tailgate open position) or the "open" switch above the number plate (operated externally to open tailgate).  All appeared normal and nice and dry. The close operates well with no apparent stickiness.  

The button below dash (near bonnet release catch) seems to operate normally, although I have not checked that for moisture.

No fault is apparent on OBD scan.

I'd appreciate any pointers on how to proceed.

Morning Tim

Welcome to the Forum

Oh Err that is a bit of a weird one and a bit of a worry for you

It has to be a faulty switch or possibly a short in the loom 

I would start with all switches and connectors clean dry and clean with a good electrical contact cleaner then seal the connection (I have had success with plumbers tape) If that doesn't do it check the loom and lock carefully

Dave