Hi lilacben,
Has your Grand Daughter maybe seen something on TV recently, perhaps was a bit scary for her, and is re-acting to that? or maybe read a book or story that was a bit too adult for her and was abot noises and knocks etc?.
I would just do as your daughter is doing...play it down and re-assure her that nothing at all is there and maybe have a bit of mid-evening fun before bed, such as a game of cards, quiz games or a board game, even a funny dvd, so she is relaxed at bedtime.
We have personal experience of ghosty's and sacry objects with our two youngest kids when they watched an episode of Dr Who, which was broadcast at about 6.15pm, with NO announcement warning at the beginning that it had really scary content in it, and it featured things in it called Weeping Angels.....scary statues that if you blink, wil come nearer to youand eventually take you away.....they both screamed their heads off and Mrs bear turned it off straight away......even though it took about 3 weeks for them to forget about it and sleep properly again......The Weeping Angels returned in later episodes and they STILL at a few years older, wont watch it!
I hope you getit sorted and your lovely grand daughter gets over it really soon....it may be just a phase....I would advise your daughter to monitor it and see how she responds in a couple of weeks...Christmas is so near now and hopefully the excitement of it all.....decorations and presents....Father Christmas ( yes, he DOES exist imo lol ) will take her mind off her worries.
Best of Luck, sweetheart
:0) x