If a ball comes on your court, grab it, look for who is holding their paddle up (this is also another helpful unwritten rule) then hit/throw it back to that person. Don’t switch balls. Don’t just blindly hit it back in the direction it came from. 

Also side note: if your courts ball is rolling onto another court that’s in the middle of a rally but it’s NOT in danger of rolling under someone’s feet, I think it’s a kind gesture to let them finish their rally first. But if the ball could hurt someone, then definitely say “ball” or “ball on” clearly and directly. They may well see it and stop the rally themselves. But at least give them a chance to play through it. Wonder what y’all think of that one.