Measure where the leak is downstairs in relation to the outside wall, then use that measurement upstairs from the same wall to figure out the approx location in bathroom, i.e. if it's your bath or toilet, I bet they have thought it's your toilet when it's actually your bath.
Had a leak on our new build too, I took the plug off the bath drain and noticed one of the 'o' ring seal was only partially located, so I took the bath panel off (to which I was surprised to see was supported at the top with slices of empty silicon tubes squashed to stop the panel moving about at the top !!!), pulled the pipework apart and refitted seal, leak sorted.
Then a few days later another leak appeared from what appeared same location, the 'builders plumber' traced it to the toilet u bend seal being pushed off when originally fitted!.
This is in addition to my kitchen sink trap u bend compression connection not being tightened and falling off when a sink full of water was draining! , yep sink full of water in my new kitchen cupboard!. All within a few days of moving in!, nothing is checked at all.
Just over two years later my four toilets now all have failing flush valves.......useless new builds!