Maybe someone had some other module or something plugged in like TV tunner/DAB(depending on region).

Did you try tracing it ? If you lift up the back cargo mat then on the left side there you can take out the plastic insert covering the bluetooth module. It's only 3 10mm plastic nuts. Then you get idea.. if it's already plugged into a module back there depending into which one it could be an antenna or just a wire to something else the previous owner had.

If it looks like it goes up front could have been something the previous owner kept being everything is working fine.

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2006 X5 4.4i