Cracks: simple small, even hairline, cracks will expand with temperature fluxes, progressively allowing for more water to infiltrate. Look for water stains, and test surface integrity by pressing firmly on areas that maybe bubbling underneath.
Nesting: animals, inspects, and mold are all looking for places to live too. With small holes and cracks expanding, this provides shelter for them to live/hibernate.
Separation: when the above superficial issues become structural concerns. The walls will begin to crumble, exposing the inner walls to the outside elements. At this stage, the next blog you read will be on the topic of pest extermination, mold remediation.