No particular order
1a. Press Location in Shinko, gives message you need key in Montparnasse
1b. Press Key in Montparnasse, removes it from map, adds it to inventory. Gives instruction for the Village Terazza.
2a. Press Location in The Village Terazza,
2b. Key in Italy in Paris, code is 1861.
3. Press Location in Marais.

Several categories of objects

Location objects: Loaded on first load, but you need item to open them.
Key object, you need to find them. Marker appears at certain level of zoom.