That's what had been done by the previous ******* brothers reno done on my old place.
I didn't notice at first, but the building report picked it up and mentioned it was illegal to tile over them (in NSW).
We knew they were there, as we could see the pipes outside, and peering in through the ventilation bricks, you could see the exterior drainage pipe go in under the house, turn vertical, then go up against the floor.
When I jackhammered up 2 layers of tiles & concrete, I uncovered them both.
In your case, I'd be looking to see if they had floor drains - by checking the pipes underneath (if it's accessible).
You can also retro-fit floor drains, as long as it's a suspended slab or sheet flooring. Suspended slabs need a new hole core-drilled or jackhammered thru - we did this on the previous place as we wanted to move the location of the floor drain away from where you stand at the vanity - it was rather uncomfortable!
We moved it closer to the centre of the room, so the tiles only had a gentle fall from all-round, and so it was closer to the bath & shower - the 2 more likely 'flood' sources.