Uncommon for them to come off unless improperly installed (easy to do). Sorry. I've done hundreds of brake jobs (used to race) and can see how it could happen.

Dealer is your best bet here. Getting new ones isn't a bad thing for these.

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Kirk
Las Vegas
2016 X5 40e Mineral White/Black Dakota Leather, ZLL, ZCW, ZDA, ZDB, ZPP, multi contour seats, rear side window shades, HK stereo
2011 E90M3, 6-speed manual