Discussed already were the "customs" surrounding fasting when a Sefer Torah falls the floor:
The earliest source is the Shu"t Mishpetei Shmuel 12 quoted by the Magen Avraham 44:5. He bases it on the gemara in Moed Katan 26a that says you need to fast if you see a sefer Torah burnt in your dream. The Igros Mosheh OC 3:3 holds that according to halachah, most authorities hold that only those who drop it need to fast, but there is a minhag for those who see it to fast as well.
What about if someone would see an entire (presumably rather small) Aron Kodesh fall to the ground with a Sefer Torah inside of it: Would this person also need to fast based on the minhag mentioned in the other question (quoted above)? I suppose the question could also be asked if the same would be true for someone who sees a Sefer Torah fall to the ground when it's inside any other structure, box, covering, etc. However I'm specifically interested to know about the case of when it's in the Aron Kodesh (perhaps such a case would be more lenient since it's a bigger structure).