[bimmerfile]

[QUOTE=bardofoc;10263882]While washing my 128i yesterday afternoon, I noticed an aberration on my rear bumper. In the right hand corner, there appeared to be a bulge protruding out from an otherwise smooth and undamaged finish. I was surprised by this anomaly’s sudden appearance. I initially thought someone had dinged my bumper, but that scenario didn’t seem right. Like I said, the damage is in the form of a bump or a ‘bubble’ that had been caused by an impact inside the bumper. If this had been caused by a collision between my car and another object, one would expect to see an indentation in the bumper and not a rise. I was then certain this had been caused by a malfunction or a defect within the car. But, now, I am not so sure either way. There are three reasons why I am not convinced this was the result of a collision.
1) A lack of indentation at the point of impact
2) Other than a slight crease around the bubble, there are no cracks, holes, scratches, or scrape marks
on the bumper
3) Just as important, there are no paint marks that rubbed off from the car that collided with mine.

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OP;
Don't bother going to the dealer - this is not a defect (IMHO).
I had the exact same bulge on my back bumper on my 2008 128i 2 years ago.
What happened was that, while my wife and I were shopping, a lady backed into the back of my 1er. She didn't notice it, but a bystander stopped her before she could get away and told her what she had done.
She was in the process of writing me a note to put on the windshield when we came out.
I noticed the little bulge (identical to yours) and told her I'd get it checked out at the dealer for repair estimates. BTW, other than the bulge, there were no marks on the paint to suggest that my car had been hit by another vehicle - the paint and finish on the bulge were still pristine.
But apparently there is a rebar (I'm not sure if that is the correct nomenclature) which is behind the plastic bumper which deploys which had some damage and caused the bulge to appear.
The bumper does not have to be replaced, but the rebar and bumper will need to be repaired.
The bumper was brought back into shape with some sort of heat process. The entire repair cost $850 Cdn - which the lady paid for.
Your damage looks identical to mine, so I suspect someone also backed into you at a low speed and may never even have noticed the bump.
BTW, you need to get it repaired as the integrity of your rear bumper has been compromised from a safety perspective.
Good Luck.