Rainboe;
I don't think you'd get cameras in washrooms past the passenger groups, not to mention the FA's etc.
I'm assuming you've considered and dismissed the effectiveness of smoke detectors?
At least in the 320/340 series, the warning sets off the Master Warning and brings up a red ECAM warning -I assume the same level of warning is on all transport aircraft flying in Europe, NA, Australia NZ etc - (iow, excludes Africa and some parts of Asia) - there is no doubt about the warning, and the FA's inspect immediately as an SOP.