If anyone in my group is moving ahead to do something alone, two of us remain engaged with what is going on and talking with the player if they're trying to figure something out (though that knowledge remains OOC, up to him to figure out how his character might discover it). The fourth player definitely zones out. But that player zones out no matter what and is definitely the least vested in the game in general. Like... if it's not his turn in combat he's on another planet.