Treasures are located all over Constantinople, in lootable gold chests. Some will contain money, others rare bomb ingredients. Their exact contents is to an extent random, but there is one easy way to make sure you get them all if you really need the extra money.
Buy the maps from the bookstore. They may initially seem expensive, but they pay off in the long run as the value of all the chests in each area far exceeds that of the map.
There are also golden Treasure chests in the hidden areas, too. However, these can be a lot easier to find, as they’re generally located at the very end, next to the Eagle Sense gateway.