Once-off each time, sounds like a very minor mechanical play (like the brake pad fitment suggested above). As long as the brakes are working properly and you aren't living with a noise during operation, I would just tuck this in the back of my mind and leave it for now.
Check out the various brakes threads for possible solutions going forward (no pun intended

):
aftermarket pads on well bedded rotors will solve almost all manifesting brake shudder (should that arise; not borrowing trouble for you here).
As for hearing a grinding sound: the manual does say that noises during braking with "high performance Brembo brakes" is normal and to be expected (mine have been quiet for over 44K miles, still on the OEM brakes). But your "grinding" sound happened after a lot of rain, so is probably only grit thrown up from the road: it could be stuck in there, like gravel does sometimes: if you went through a big puddle that would increase your chances of getting crud stuck in your calipers: should be easy enough to tell by looking, and you can pick it out by reversing slightly first to loosen up any jammed foreign matter. If that doesn't eliminate the "grinding" sound, take it in and get it up in the air so that they can clean the calipers/pads properly.