What is the ruling on walking or sitting on an impurity of unknown status (dry or wet), and does impurity transfer thereby?

The default is that impurity does not transfer and purity remains. So, if we doubt the impurity of something, we assume its original state, which is purity, for doubt does not remove certainty. It is advisable to disregard obsessive thoughts and not pay attention to them, as this is an effective remedy for them.