Was the mechanic just lazy?
I don't know why this is in the commuting forum but I guess it's because its my commuter.
I took my bike in for a full tune up and cleaning the other day and after I got it back, I noticed my disc brakes felt a little weak. So I took it down an empty street, got it up to about 25mph, slammed on both brakes and rolled a good city block before stopping. Did that a few times and it's all good now.
I guess the mechanic left a lot of grease or something on my discs? Who would grease up my discs!! My guess is he forgot to clean em..