To unlock the "Bonus Miles" trophy in Life is Strange 2, you must find optional collectible 3 in Episode 1. It's labelled as the third, but the second, third and fourth can actually be picked up in any order.
After your first night in the woods you will be approaching a gas station. To get this collectible, head into the gas station. Immediately turn right from the doorway and look back out of the window towards the forecourt. There's a pile of stickers on the windowsill. Take one to unlock the trophy. Make sure to get this before setting yourselves up on the bench outside to study the map.
Screenshot for reference: