Drains, sewers, tunnels, and other alternate access points must also be considered in the design of the PPS.

(Source: Senstar Corp.)

include such structurally framed openings as air ducts, tunnels, crawl spaces, skylights, sewers, attics, chimneys, and exhaust fans. Essentially any area where an adversary may be able to crawl undetected and bypass security layers need to be included in the design of the PPS.