Is it possible that you have a squeaking kind of after market brake-pads?? There are several suppliers of after market brake-pads, some are better, some are worse and some have the intention to squeak every time you use them...
Metal-metal sound can be caused by broken pads as well; it is possible that the pads still look good, but are cracked inside.
I have even seen after market pads that contained metal parts (such as a little pipe), such as a big time fault during the production of the pads. Did the mechanic take out all the pads or did he just have a quick look?
I don't know what sound you are hearing, but these are just some suggestions...
You can check whether the sensors are put in by looking at the (front) calipers. There has to be a little cable at the outside of the calipers (the front side of the car) that comes from the outer pad and goes to a plastic cover on the caliper. You can see approx. 2cm cable if you look at it.
with a minor 600+k