Strongboxes are a kind of Secret in Tomb Raider (2013) and Rise of the Tomb Raider. They can be found all over Yamatai and Siberia. Strongboxes are big metal crates that are locked and can not be opened without a tool.
Tomb Raider (2013)
Rise of the Tomb Raider
In Rise if the Tomb Raider they contain Weapon parts. Collecting three parts of the same weapon will unlock an new weapon. They are red and again Lara has to use her Axe to open them. Some Strongboxes are more secure; here she has to use the Lock Pick she acquires in the Mission Communications Breakdown.
Lara opening a Strongbox in Tomb Raider (2013)