FBI cannot even look at your phone lock screen without a warrant, rules judge
Precedents are interesting:> Bond v. United States, 529 U.S. 334, 337 (2000) (concluding Border Patrol agent searched a bag by squeezing it)> Arizona v. Hicks, 480 U.S. 321, 324–25 (1987) (holding officer searched stereo equipment by moving it so that the officer could view concealed serial numbers).The idea seems to be that pushing the power button is what constituted a search, not taking a picture of the lock screen. Presumably if the lock screen was somehow already on at that point (which normally it wouldn't be) then it might be fair game to take a picture without a warrant.