Good link Mike (albeit a little large).
Robert…in our state we have regs known as VIAQ (Ventilation and Indoor Air Quality) http://apps.leg.wa.gov/WAC/default.aspx?cite=51-13&full=true .
Essentially it requires some means of drawing fresh air into newer “airtight” homes on a regular basis. In your case, it probably controls a damper on a duct to the exterior that allows fresh air to be drawn into the return plenum. On other houses you might find a “keyed” wall timer in one of the bathrooms or laundry controlling the exhaust fans (often combined with small vents at the windows).
Methods vary and can get pretty complicated and there’s really no way of testing the efficiency of these during a normal home inspection. Homeowners will often set these to absolute minimum times or, in the case of the bathroom fans, remove the keys completely as the fans coming on “annoy” them. Point the timers out to the client, explain the general purpose, and suggest they try to get the manual from the seller.