The blank note is found in shaft 13. You'll have to climb through the spider tunnels there, and in a locker room you'll find in a locker (obviously) a blank note. After you have that, go to shaft 12, there you'll find a train cart, cut the chains holding it in place with the boltcutter from the tool shed, and push it down the slope. When you walk through the crashed wall go right, go into the room, turn on the UV light, double click the blank note in the inventory and use it on the UV light. Now you'll find the answer. Good luck!

My problem is somewhat different. I had already found the blank note and deduced that I have to enter shaft 12 and I cut the wires preventing the wagon to go down and pushed it. But the wagon is stuck (de-railed?) halfway and I have been trying to find a workaround for the last 2 hours. Have I missed something or should I do something else to make the wagon descend completely?

I found a blank note in shaft 13. I found closed door in shaft C. And u say i have to go to shaft 12 to use blank note on UV light but i cant cause door to it is closed! so only where i can get in is some kinda of oil plant room, close to shaft 12. and what to do there? i go ahead and i see big ventelator upstars, also an attached ladder but i dont know what to do there!