Sleeping Dogs

There is a total of 28 open world Events. 17 of them are Drug Busts. The rest of the Events are unlocked during your time in Hong Kong. These only appear on your map when you are very close to them. Some of the events only appear during certain times and days. If you fail one, don't worry you will be able to replay it again. These will appear as different colored shields:

17 Drug Bust (Light Blue)
4 Shoplifters (Yellow)
3 Hit and Runs (Red)
4 Junk in the Trunks (Orange)

*Drug Bust: They are shown on the map as blue shields (lighter than the case shields). To complete the event you need to defeat the enemies, then hack the camera nearby and then go back to your apartment and press Y on the TV select one of the camera's that say Triad Activity and Press A on the dealer to have him arrested.

Here is a map of all open world Events.

All Events:
17 Drug Bust (Light Blue)
4 Shoplifters (Yellow)
3 Hit and Runs (Red)
4 Junk in the Trunks (Orange)

Do half and the achievement will be unlocked