Measuring humidity through ceramic tile to deduce a course of action
Water penetration is usually destructive, such that damage is detectable without fancy instruments. The tile grout on the outside of the curb would show signs of being wet and/or you might see efflorescence from evaporating moisture. As well as loose tiles, wet floor, etc.

If the shower curb is indeed sloped towards the outside of the enclosure then it was not designed correctly, which is a concern. Put a level on it. In the end, if it's been like that for some time and there are no signs of damage, it may not be something to worry too much about. The detector is not the "be all end all", a few years back inspectors did not even use them.