I got this on Seattle Day One. You can enter both the dome and courthouse without forced story progression occurring, so there is no worry of missing this if you trigger a story action. As long as you do it all before continuing with the objective, it will be fine.
When I unlocked this trophy my map looked like this:
Where my player icon is on the map is an unmarked location you can jump over to and loot a firetruck, then abseil down to loot another truck. Unsure if those are needed for this to unlock since it isn't marked, but worth mentioning.