If you're having trouble finding a treasure chest in Monteriggioni, chances are that it's in this video.
The purpose of this video is to show you where the secret areas and chests are in the Assassin Tombs. This video has two secret chests ( to my knowledge).
The first one is at 1:15
The second one is at 3:55 ( strange way of reaching the chest, didnt know about the proper way)