After completing the Before Berinsfield ad lab you can find this bonus cache. It's quite near to the suggested parking area. You need the "Adventure Lab" ap.
You will need to insert the numbers you find into the simple formula:
N 51 39.ABC W001 10.DEF
A little stealth will be needed if muggles around, around 3 to 4 feet up. Vegetation provides camouflage! Tweezers might be useful so you can sign the log, with the pen you always carry, of course 😬!!
Magnetic. Not on the ground.