Mystery leak from brake caliper. What to do?
Sounds like a bad piston seal to me. Could also be any seal from around the calliper area, split line, cracked calliper.

Assuming these are newish and you've not bashed them up this should be a warranty claim. This will almost always have to go through the place you bought the bike, but there are exceptions.

You should email SRAM, the maker, and the shop to see who wants it back and see if the whole bike needs to be retuned or just the failed brake.