I found the way to trigger a hidden Sapper Master quest.
You need to use Metal Detector in marked spot of Sentry Bailey.
The rest is pretty easy.
You pick up mine for a quest item that you need to deliver to Sapper Master in East Siauliai Woods.
Note: I was not able to pick up the mine with mouse click. I had to use keyboard.
AFAIK this quest wasn’t known until now.
It’s nice to know there’s still things in this game left to discover.
