People suffering from sick building syndrome are often misdiagnosed, and may be suffering from multiple chemical sensitivities that further complicate making a diagnosis. Even if no one sees, feels or smells anything, the allergen or substance may be there. This occurs at both ends of the spectrum: either in an airtight facility or one with poor ventilation.
Be on the lookout or chronic fatigue symptoms, unusual pollen or mold reactions, all of which could be from living in an environment where the toxins are trapped inside your living space.
In these circumstances, having the air tested for pathogens (mold, benzene, formaldehyde or particulates) is a wise move.