My first reaction is that residual grease would show up quite quickly after doing the brakes and rotors. Funny that it took 25k to appear. But a blob of grease, oil or whatever that got onto a rotor later could contribute to the problem.
Question- is the ridge between the bad spot and the rest of the rotor surface high enough that you could catch a finger nail on it? Just trying to imagine what it looks like.
This is what I conclude: You didn't do anything that contributed to this problem, at least as far as the original brake and pad work. If you had, my guess is it would have shown up a lot earlier.
One quick thought- did you or anyone lube your wheel lug bolts? Maybe some of the lube got flung onto the rotor.
Pull the disks, yes both, have them checked to see if you can turn them. If so do it, around here I get them turned for anywhere from $15-20 a rotor. Reinstall everything and chalk it up to one of the great mysteries of owning a car.
'03 E320 Wagon-Sold
'95 E320 Wagon-Went to Ex
'93 190E 2.6-Wrecked
'91 300E-Went to Ex
'65 911 Coupe (#302580)