The sensor says ultrasonic but may also detect heat signature or something getting heated makes a noise. It's designed to not trip from a slow change but I suspect when it does trip the sun lines up just right to cause the problem.

Why didn't it to before? Because the sun's path changes though the year and didn't line up just right.

I'll bet if you watch how the sun hits your car you can see some magic alignment. You may be able to resolve as simple as a sun shade.

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2011 E70 • N55 (me)
2012 E70 • N63 (wife)