Difficult. It might be that they can still smell the wee - cat's have a very sensative sense of smell. if they were doing it because it was time to get up I would have thought they would be meowing to wake you. perhaps you should just ban them from the bedroom especially if they are using the litter tray as normal the rest of the time.
Maybe they're just too lazy to get off the bed!

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If cats could talk they would lie to you