40 CFR 312.31: The degree of obviousness of the presence or likely presence of contamination at the property, and the ability to detect the contamination by appropriate investigation.

(a) Persons to whom this part is applicable per § 312.1(b) and environmental professionals conducting an inquiry of a property on behalf of such persons must take into account the information collected under § 312.23 through 312.30 in considering the degree of obviousness of the presence of releases or threatened releases at the subject property.

(b) Persons to whom this part is applicable per § 312.1(b) and environmental professionals conducting an inquiry of a property on behalf of such persons must take into account the information collected under § 312.23 through 312.30 in considering the ability to detect contamination by appropriate investigation. The inquiry of the environmental professional should include an opinion regarding additional appropriate investigation, if any.