I helped a guy repair his mountain bike that had a bent derailleur hanger. The derailleur actually was slammed into the spokes momentarily. Checking the spokes I noticed shallow indentations and abrasions where the derailleur got hung. They were about 1/3 of the distance to the rim from the hub on the drive side. He did not replace the spokes and as far as I know he has not had any problems. But he was running 36 14ga Sipams. I wonder if the same type accident could have precipitated your current problem...

Further note: Having a back up wheel set for your favorite bike has been a real luxury for me...