Does this happen to anybody else. When it rains and I'm parked outside does anybody else breaks squeak or put out some high pitch noise for the first couple of feet while breaking. It's quite embarrassing when I'm pulling up to the stop sign and it squeals so loud peoples head all turn.
I was getting something like this recently, my Rear pads are getting low, (I realised my Discs needing changing also which I didn't have at the time), so I added a slight champfer of the remaining Pad leading edge (Manufacturers one worn away) reassembled them adding Brake lube to the channels the pads slide in and generally got rid of all the road salt and old brake dust that was hanging around them and it settled the squeaking down.
A few thousand miles and i'll be replacing rear discs and pads.
You don't mention the mileage on the pads, but I would guess it just needs a clean and targeted lubrication, if it doesn't need new Pads/Dics yet.
I would normally think it was the milage but it's still under 5k and it only happens when it rains and only for the first few times of breaking. my best guess is moisture trapped between the breaks after the rain makes it squeak and after burning it off by breaking for a few feet it burns off. but those few feet sound terrible.
Does sound odd if only when initially wet, or maybe they forgot to fit the Anti-Squeal shims...
I would think that when the brakes are wet they kind of hug and then let go in a rapid succession until dry, causing the squeking between the piston and the pad. So a bit of coppergrease would cure it