what details are given as hints to an alert reader that the one chance of finding the keyhole is about to resent itself?

When they arrive at the mountain and decide to stay far away from Smaug's entrance, Bilbo and the dwarves move around to look for the secret door he remembers from the map. They figure out where the door must be located, but they can't open it. Even hitting it with picks and axes and trying to break it down they aren't successful

Discouraged, Bilbo sits down right outside the door as a thrush appears and perches nearby to peck at a snail with its beak. Suddenly, Bilbo remembers the riddle written on the map. The group then patiently waits for the sun to set, the dimming sunlight to flicker on the door where the rock will fall away and reveal the keyhole. Then they enter in the way that follows;

"Then Thorin stepped up and drew the key on its chain from round his neck. He put it to the hole. It fitted and it turned! Snap! The gleam went out, the sun sank, the moon was gone, and the evening sprang into the sky.

Now they all pushed together, and slowly a part of the rock-wall gave way. Long straight cracks appeared and widened. A door five feet high and three broad was outlined, and slowly without a sound swung inwards. It seemed as if darkness flowed out like a vapour from the hole in the mountain-side, and deep darkness in which nothing could be seen lay before their eyes, a yawning mouth leading in and down."