Small CCTV cameras, discreet, aimed to cover toilet, movement-sensitive , comes complete with SD memory-card slot and battery-power........attached by clips or suckers, could be placed by CC before boarding.
As it's not a fixture,no need for certification and all the crap attached to that......cost~£100-£300 each....less than a month's wage for a Hostie, to cover all the toilets and probably most of the cabin.
In case of incident.....review footage for last entry or two , to the bog.....99.5+ of pax immediately eliminated from enquiry.
Cheap, simple, cost effective....Oh, and like a CVR, they automatically overwrite the card, so you have a rolling record of last few hours, depending on frame-rate and trigger-threshold.
but that's too easy, isn't it....no jobs for the boys there.
Antisocial pax could always be shown the door at 10K feet, before resuming cruise
