Shekalim, Chapter 6, Mishnah 2. In the previous Mishnah we learned that the household of Rabban Gamliel and the household of Rabbi Chananyah, the deputy Kohen Gadol, would bow one additional time in the Beis HaMikdash, beyond the thirteen regular bowings, at the spot where they had a tradition that the Aron HaKodesh had been hidden away. Our Mishnah shares with us the incident that gave rise to this belief.
The incident in the Mishnah:
"Ma'aseh b'kohen echad shehayah mis'asek" - It once happened that a certain kohen was occupied with his work. The Mishnah does not specify what he was occupied with, only that he was busy at his task. As we mentioned, this refers to the Chamber of Wood, the room in which the wood was stored, and he was inspecting the wood for worms: wood found to have a worm in it is considered blemished and unfit for use on the Mizbe'ach.
"V'ra'ah ritzpah shehi meshunah mechavroseha" - In the course of his inspection the kohen noticed the floor, and saw that one spot in it was different from the other tiles, from the rest of the marble tiles.
"V'lo hispik ligmor es hadavar" - He went and told this to his fellow, but he did not manage to finish his words and point out the exact spot, for he suddenly died.
"V'yad'u b'yichud shesham ha'aron ganuz" - At that moment they knew with certainty that this was the place where the Aron had been hidden; and this was precisely why the kohen died so suddenly, without being able to reveal the secret of exactly where within that room the Aron's hiding place was located.
A closer look at the details of the incident:
It stands to reason that the floor tile had been moved at that particular moment on a temporary basis, for otherwise the others would have been able to find the spot on their own. In other words, these were special circumstances in which that kohen was able to recognize that the tile had been shifted from its place, while to others it was not noticeable at all.
In summary: In this Mishnah we learned the incident that lies at the root of the tradition regarding the place where the Aron was hidden: a kohen who was busy inspecting the wood in the Chamber of Wood noticed a floor tile that differed from those around it, and died suddenly before he managed to reveal the exact spot - and from the suddenness of his death they knew with certainty that the Aron was indeed hidden there.