From Fort Stevens Historical Site & Military Museum parking lot ($5 day use fee), find the Coal Yard. If you’re unfamiliar with the area, visit the museum and pick up a Guide to the Historic Military Site.
Once you get to the Coal Yard, let your magic machine lead you to GZ.
When you get close to GZ, you will have a short but somewhat steep climb.
It’s a nano but probably won’t be what you expect.
The log is in the vial. Bring your tweezers. Bring your pen.