WillCAD

Background checks are slanted heavily toward looking for past criminal convictions and affiliation with any known terrorist groups. They don't delve as deeply into one's past as a security clearance does, and they certainly don't include any mandatory mental health evaluation. As such, it is not only possible but highly likely that the background check would completely miss any mental instability if it had never led to an arrest.

I don't know whether a federal background check even looks for past commitments to mental institutions, since such a commitment is considered health-relation information. It might be protected under ADA. It might also be simply considered irrelevant to those paranoids among us who are tasked with looking for terrorists and foreign spies; those who might become terrorists or foreign spies; those who have associated with terrorists or foreign spies; or those who once caught a whiff of a terrorist's or foreign spy's cologne from across a crowded room and are thus considered "under the influence" of said terrorist or foreign spy.