I believe that unfortunately the fourth amendment aspect, regardless of Smith and third party doctrine, will fall flat because of another line of cases, namely the contraband cases. Caballes, Jacobsen, Kyllo, all of them tend to point in the direction that if the "search" reveals <i>only</i> the presence of contraband, that it's not in fact a search at all, and the Supreme Court has tried as hard as it can to avoid creating such "quasi-searches". Florida v. Jardines seems to have been decided primarily on the place being "searched" (the home) and while a phone is probably an "effect" under the Fourth Amendment (Riley) it's far from an easy case and in fact may be a difficult one because they will argue that it's not a search at all.