August Smart Lock works even without power or Wi-Fi service, Mr. Behar said. And if a smartphone is lost or stolen, he added, the user can alert a service that will deactivate the August app. But to do so requires logging in to an Internet-enabled device. And as with losing a mechanical key, there is still the hassle of tracking down a friend or relative to let you in (or using an old-school key, which defeats the point of the technology).