Hi.
If they should be still visible and on the edge of the screen, i think i would place some triggers as child on the camera on the edges or near the edges.
these triggers can set a bool to true when object is inside the trigger and to false when not anymore inside trigger.
then something similar for the fov, if inside fov set bool true if not false.
Then have a bool all true to check if both are true, if both true....BOOM!