At the Fort Stevens Historical Site & Military Museum parking lot ($5 day use fee), walk or bike across the yard in an easterly direction. Your destination is the Plotting Room.
If you’re unfamiliar with the Ft. Stevens Historic Area, stop by the museum and ask them where the Plotting Room is.
It’s a fairly steep incline. Might be slippery if wet.
It’s a nano but may not appear to be what you expect.
The log is in the vial. Bring your tweezers. Bring your pen.