I am looking after a spy to fixed under my timberflooring on site, to be able to proove that the dammage were made by water.
A client call me 2 weeks ago, I checked at his place the flooring, in the corridor, near the bath room, the planks is being warp. I guess that an accident happened, and they waited probably 2 to 3 months to call, when everything is dried. Telling me that the job was badly done.
I heard that a company in Germany produce those spy to be put under the timber flooring secretely. When water comes into contact with it, the colour will change. This will be a good proof towards the client, that lot of water come into contact with the floor. The lady wants free repair.

If it is sufficiently dry how can any device detect the moisture? And who is going to snaek into someones house to plant it anyway?

Any inspector worth his salt can tell if a floor was water damaged. Have her call an independant inspector. Tell the lady you will pay for the inspection if it turns out to be your fault.