My guess would be (was) about the center of the dash or possibly the same module as the card reader behind the two cup holders in the center console. That would mostly centralize the sensor for the doors but leave the trunk a bit far for a short ranged one.
Just looked up the EPC... found the bluetooth module 1508179-00-E (and F)... then looked up the service repair for the image showing where it is...
View attachment 968578
Its the smaller rectangular one below(?) the touchscreen. I'm not actually sure if its below the touchscreen or maybe on the driver's side under the dash.
But! Doing a search for bluetooth on the service manual shows there's a bluetooth antenna in the rear bumper area (which is also shown in the image above, just didn't know it).
So, I have no idea why it behaves the way it does, if its supposed to have an antenna there. Maybe its faulty or something... I dunno.
edit: could be the one in the dash is for the TPMS, but that still leaves the one in the rear and the weird behavior.