Shower floor tiles or grout leaking water

Hi all.
Just moved into a friends house, and the upstairs shower is leaking, badly!
From the top, there is a PVC white drain, that goes down into a deep U bend, that then leads off down the walls somewhere.
We have removed the drywall (ceiling) from directly below the shower to gain access. All we can see is the bottom of the drain, and the ubend followed by the pipe disappearing into the walls.
The shower floor itself appears to have been recently re-done. Not that I am an expert in any way, but it looks pretty new.
The floor is a mosaic of fairly small non-skid tiles, approx 2" across. There does not appear to be anything other than cement or dark grey grout inbetween the tiles, and at the wall/floor joins.
Is there some kind of sealant we can apply to the grout? It does not appear to be cracked anywhere.
I have made sure that the tap fitting itself is not leaking by pouring water from a bucket directly onto the floor of the shower. (It rained downstairs from around the PVC drain).
Do we rip it all up? Or is there an easier fix?
I thought there was supposed to be some kind of catch tray under the shower floor? Am I wrong?

maybe the area around the drain is leaking. remove the drain and use plumbers putty to seal around it. re-seat the drain remove the access putty and see what happens.

Code requires a shower pan or liner to be installed, then 'bedding mortar' is installed over the liner (20 mil PVC) or pan to build the 'slope', then the tile is installed on the mortar, then grouted. Grout is not waterproof, no sealer will make it so. If the liner/pan is leaking badly from around the drain you may have an easy fix by removing (all) of the PVC drain components siliconing the hell out of it and putting it back together. Let it cure for a few days and then plug the drain itself, and fill the shower with six inches of water. Leave it overnight. If it's not shown ANY sign of leakage from underneath, you might have gotten lucky. If there is any leakage then the hydrostatic pressure has shown you another leaky weak point, and you'll have to rip it ALL out, and start over from scratch to properly fix the problem...

Good luck!

Thanks all, turned out it was a torn pan! Previous install was a joke, no mortar under pan (pan was 'floating' over the wooden frame underneath!
Luckily insurance covered most of the replacement, had to rip out all shower times, in order to match again.
All sorted now.