In buildings on historic sites, this can sometimes have an archaeoligical explanation, as buried walls and pits can produce hard and soft spots. York Minster is a famous example.
Settlement can occur at a very early stage in the life of a building and can be live or expended.
There are a number of sophisticated techniques for measuring rate of settlement ranging from tell tales to laser technology. Tell tails are glass strips carefully positioned and secured at each end over cracks. Drift is the gradual displacement of the fabric of a structure caused by the expansion and contraction which is the natural result of temperature change.