To add in a wiki summary:
It is mentioned as the place the Ark of the Covenant may have been moved after being in Beth Shemesh (1 Samuel 6:21-7:2). About 60 years (2 Sam 6) afterward, the ark was moved to Jerusalem and placed in a tent outside the palace of David.
https://en.wikipedia.org/wiki/Kiriath-Jearim
As far as the location of the Ark, I've always had a soft spot for a dumb pun angle.
That is, that it would be located in Debir (aka Kirjath-sepher or Kirjath-sannah), because debir is
"the holy of holies, the innermost room of the temple or tabernacle"
So yeah, where *else* would it be? ;-)
I haven't read the article, but there is a theory it is located underneath the crucifixion site. The blood of Christ, his body having been pierced, flowed through a crack in the earth, caused by the earthquake, anointing the Ark seat.
Then there is the Holy Grail that could have more easily been used for just such a purpose.
It was also in the writing that the prophet, having received an oracle, ordered that the tent and the ark should follow with him, and that he went out to the mountain where Moses had gone up and had seen the inheritance of God. And Jeremiah came and found a cave, and he brought there the tent and the ark and the altar of incense, and he sealed up the entrance. Some of those who followed him came up to mark the way, but could not find it. When Jeremiah learned of it, he rebuked them and declared: "The place shall be unknown until God gathers his people together again and shows his mercy." (2 Maccabees 2:1-7)
I think the Ethiopian theory is the most likely.