There are 12 Billboards that you can hack around glass - each of them requiring progressively more advanced freerunning or exploration to scale and hack.
There are 7 Intervention "missions" which are more like random events where 2 K-Sec guards are threatening another citizen. This video shows the location of all of them:
You only need to do 15 total combined of either Billboards or Interventions. Though despite doing all Billboards, I had to do 4 Interventions to get this to pop.