Lockboxes are collectibles in Sleeping Dogs.
The lockboxes can contain Hong Kong Dollar, weapons, or clothing. Lockboxes may be guarded by enemies, which Wei will have to defeat to access the lockbox. Lockboxes may be unlocked, or they may require a combination to be put in.
Unlocking all the lockboxes in North Point will award the player with the 'North Point Scavenger achievement/trophy, opening all the lockboxes in Central will reward the player with the 'Central Scavenger' acheivement/trophy, and opening all the lockboxes in Kennedy Town and Aberdeen will reward the player with the 'West End Scavenger' achievement/trophy.
Completing the mission 'Make Ilyana Sweat' will show the locations of the lockboxes on the minimap.
If you defeat a bunch of tougher enemies (e.g grapplers or brawlers) then there will be a bigger reward in the corresponding lockbox. The money you can find can be up to HK $20,000. You usually get a HK $5,000 money stash if you get a firearm from the lockbox also. The most common are the HK $5,000 lockboxes and then the HK $10,000 ones. The lockboxes glow blue and flash slightly so they can be easy to find. If you can't find it straight away after defeating enemies then have a look around the area and there should be one around.