Albert Barnes Commentary 2 Kings 12:9

"But Jehoiada the priest took a chest, and bored a hole in the lid of it, and set it beside the altar, on the right side as one cometh into the house of Jehovah: and the priests that kept the threshold put therein all the money that was brought into the house of Jehovah." — 2 Kings 12:9 (ASV)

The priests who guarded the door - The north door into the priests’ court (Ezekiel 40:35–43) seems to be intended, not the door of the temple building. The chest must have been placed a little to the right of this north door, between it and the altar of burnt-offering, so that the people could see it from the doorway. The people were not ordinarily allowed to go within the doorway into this court, which belonged to the priests and Levites only.