It was very common in the Middle Ages and probably until at least the 18th c in Northern Spain to leave a little box hidden in the altar where the priest or even the builders would leave details about the purchase of the land, or the item, or even completely random things such as their observations on a local event.
Hollowed out statues (not hard, since many of them were hollow to begin with) or architectural details were also sometimes used to hold the little box and its tiny scrolls.
Fascinating. Thank you.