Using table salt as a way to limit humidity in an unoccupied house
According to Transportation Information Service: Salt:


  At up to 74% relative humidity and 20°C, salt does not absorb any appreciable quantities of water vapor. The critical water content of sodium chloride (NaCl) is 0.5% at 74% relative humidity, which is the flow moisture point at which salt begins readily to absorb water vapor, and increases such that, at 75% relative humidity, the salt dissolves.


See this graph for better understanding.

So at high humidity, salt can absorb lots of moisture, enough to become liquid.  

When it is liquid, it will creep up the walls of the container by adhesion.

Then, when humidity decreases, the salt will lose almost all its moist, and contract, but leave residue up to the height of the former meniscus.