There are a few types of hidden items in the game; locked chests with money, praying totems, and your former martial arts teachers statues.

Locked chests
There is an easy way to find a majority of the locked money chests, because they will most likely have people you have to beat up to get to them and they appear as red on your map. These chests, like the other hidden items, brightly glow if you have not gotten to them yet. All of them are outside.

Praying totems
All of these will be outside somewhere in different "noticeable" places. A few examples are parks/gardens, near shops you can walk into, alleys, special kinds of buildings, etc. If you see one of these, use it by any means necessary because it increases your life span.

Statues
All of these will be inside a location, and a majority of them will coincide with different missions you have. For example, you will have a mission to investigate hospital files and you will have to go into the hospital; one of the statues is at the primary desk.