Possible leak / ground floor toilet

Pin pointing a leak with a moisture detector is not really possible especially if its tiled. If its a damp smell its likely a leak. If its the smell of drains then then it could be a number of reasons, simplest being water in a basin trap evaporating allowing the smell in.

Pin pointing a leak with a moisture detector is not really possible especially as we are talking about a smell. If it is dampness it will weaken the plasterboard and the tiles will start pilling off from the wall or mold will appear through the grout. And the smell of the moisture is not very strong and it is permanent. But in this case, what I will suggest to check your soil stack. If your hand basin drainage system is connected either to the toilet soil stack or shower or bath, anytime the toilet is flushed or someone taking a shower that can cause siphonage or hand basin trap to be emptied by that drained water of the bath, or toilet or shower. But because it is boxed, I suspect it is a new build. So my first approach is applicable. Secondly, check your air admittance valve may be it was not fitted properly or not working properly meaning it is also boxed, or it was not fitted. And at last, it could be dirt in your hand basin which is causing the siphonage.

I'd say you have a soil stack close by which may need have an air test carried out in sections to find whether there's a rapid drop of pressure over the test minutes period. As a measure you'll have everything place..!!, and this may involve demolition or removal of bathroom goods to access and repair to remedy the smell. Hope this helps Thanks pd

No. You are better using a micro cctv and smoke test.