The Invisible Hours

Explore the majority of the island to acquire this trophy. This will include:

Outside:
The docks
The strange tower
The hill overlooking the generator
The generator
The outhouse

Inside, first floor:
Foyer
Lounge
Library
Piano room
Chemistry room
Guest bedroom (two beds) and accompanying bathroom
Dining room
Kitchen
Pantry
*Hidden room (between Lounge and hallway leading to the Piano/Chemistry/Guest bedroom)

* Obtainable by following Flora enough after leaving the dining room.

Inside, second floor:
Vandenberg's room*
Bernhardt's room*
Edison's room*
Observatory*
Bathroom(s)
Typing room
Tesla's office*
Guest bedroom* (attached to Tesla's office, perhaps his bedroom)

* Do make sure to check any attached smaller rooms to these, as there are some (such as Tesla's workshop and the closets in the guest bedroom here.

Inside, et cetera:
Attic
Loft
Roof
Secret basement workshop
Tesla's machine room

You should actually obtain this trophy, if you're simply following Gustav, with little reason to deviate in Chapter 3. If you do not, simply wait until you follow everyone else. If you've acquired all of the cutscene chapter trophies but still do not have this, be sure to explore every little inch for the collectibles as that would surely help you to satisfy this.