OK I found it too. Its a little bigger than a bic lighter.
Its easy to see in its bracket on the frame, and labeled "REIFENDRUCK - KONTROLLE". The wiring merges with a larger harness and then to the back of the passenger rear wheel well.

I did same test as you, unplugged it and started car, got the "TPMS Malfunction Warning" and gong, flashing dash light and iDrive error message.
The plug connections looked clean but sprayed them out with WD40 and blew them dry, re-installed, started car, no error. Drove around for 20 mins and got the error 3 times. So no change.

So either this second replacement TPMS module is defective like the first (unlikely) or I have a loose wire that intermittently breaks the connection.
Do they have a diagnosing method to trace that issue if its in the wiring or CAN-BUS? This sounds difficult (expensive) when the issue is intermittent. It only seems to stay disconnected and issue the warning for ~10 secs at a time.

PLAN A - unplug and tape up the connections, and drive around to see if the gong and flashing light on the dash subside after a minute. If YES, this is how I will drive the car on some upcoming long trips. That gong and flashing light drive me nuts. One solid dash light on, I can deal with.
I will do this tonight/tomorrow to confirm.

PLAN B - Will need new tires soon, and its time to change the wheel sensors too. I guess I can HOPE there is a chance that with new wheel sensors the issue is magically resolved, but I am not holding my breath. Dealer states that would leave error codes that they did not see.

Anyone know what code / error message would be shown if the module was intermittently losing connection to one or more wheel sensors? Same as above or different?

__________________
2012 e70 X5 50i MSport "The Boss" black on black *Well Optioned*