Here are some word of wisdom from the USTA site about calling a "let."
If you have no chance for a ball and a ball rolling onto the court would have made no difference, do the right thing -- give the point to your opponents.
One can always make a case that a player could have made some play on the ball when this happens. Technically, players may call a let if the point had not been completed when the hindrance of the stray ball from another court arrived. However, a good sport knows if they had a play or not, and a good sport knows the difference between a legitimate hindrance and taking advantage of a situation.