We had a similar problem some years ago. Drains disappeared out the back of a property into a field. Southern water stated that the property wasn't connected to mains drainage as it wasn't on their map.
We hired a cable avoiding tool and signal generator. Attached a length of cable to a...
I would have set the ply dead level. Used 2 45s out of the trap to lower the waste a bit, I wouldn't use a flexi in a future inaccessible position, just asking for blockages. Then I would have gripfilled the tray down.
Not always that easy though. Nail through a pipe, isolating valve doesn't turn off, cold main stop tap inaccessible, stop tap in street concreted in. Let's stand and watch this leak bring the ceiling down whilst we wait for the water undertaker to come and dig up the stop tap.
Main issue that...
I've taken to chucking the batteries that come with the various wireless controls away and fitting duracell ones. I've had far too many call backs to wireless stats where the supplied batteries have given up the ghost after a couple of weeks.