Missing One Egg

I have been watching the two robin nests very carefully, and tried my best not to disturb the mama robins too often. Yesterday, when I did my routine evening check, I saw only 3 eggs in the nest#1, which is on the sunny deck. Since the mama robin was not there, I walked up closer to check, and found that missing egg outside the nest close to the rim of the pot. (The pot is much bigger than the nest) I didn't know what happened, mama accidentally kicked it out or something or somebody moved it. I moved it back into the nest, of course not with my hand, if mama smell human on the egg, she might abandon the nest. I was hoping that mama robin won't mind and keep hatching all 4 of them. Unfortunately, it didn't work out. This morning, I checked the nest again while mama was out there eating breakfast. There was only 3 little eggs left, and the 4th one was nowhere to be found. I checked on the deck, around the deck, around the house, just could not find it. So now, my guess is some other animal or bird attack this nest and took away one egg! How horrible! I don't know what this means to the rest of the eggs. If the "attacker" liked the taste of that egg, it might come back for more. I wish there is something I can do to protect them. So sad! And I am really worrying about their future now. Oh, I checked the nest#2 on the porch, still 4 eggs. Guess I can only cross my fingers now. One more week before the babies come out. Good luck to all of them!