At an inspection in Bellingham a few days back at a house with a "basement" I found a concealed crawl space area. This one really was concealed, no outside vents or anything to provide a clue as to its presence. However, when I found a hatch cut in the rotted-out deck, I pried it up and sure enough, there was the crawl space that nobody knew about, thought about or cared about.
As is, the condition of this crawl space alone was enough to stymie the deal on a house that had roof, electric, plumbing and structural issues. There were multiple problems in this crawl space, only a few of which include:
*No cover and strong rodent odors
*No ground cover vapor barrier
*Wood to earth contact, widespread basis
*None accessible, space between structure and grade at entry point is 6 inches
*Insulation installed upside down and rodent chewed
*Moisture level in wood 23%. Even with no earth contact wood can rot at 20% or higher
*Scraps and debris in the crawl space
Per state law, if an inspector is licensed by WA State Dept of Agriculture, he must cite that this crawl space is vulnerable to attack by wood destroying organisms (think rot and insects such as carpenter ants and the dreaded anobiid beetle) and that concealed damage may be present at this time. The mandatory recommendation goes farther and states that the crawl space must be made accessible and then inspected by a licensed and qualified party.
An unseen, unknown, crawl space is in fact more likely to harbor and hide serious problems than a more accessible substructure area.