I did the achievement after reading this specific thread - it took a few goes.
Suggest you do it nearer the end of the current update, so Riley is motivated sufficiently.
Go to park after morning school and click everything click able - go to the left, go to the right go forward and click the forest entry. If nothing head out on map to somewhere else and then go straight back to the park and try a again.
Don't panic time will not pass.
When it initially triggers, you get a cut scene, the flasher appears and then you get a reaction.
When the achievement triggers, you get a different cut scene, with the flasher and more options.
Good Luck
