Check the ground connection in your wall socket. If you have a bad ground there you might get shocks not only from the NUC but also from other appliances plugged into that socket.
In that case the current will come through the PSU.
But if you have a shortcircuit within your NUC this power will be able to "leak" to the chassi. For example through badly isolated antenna connectors (the antenna is part of the chassi).
You can also get current through the monitorcable if the monitor for some reason is bad or if the wallsockets are misconfigured regarding analog earth.
Pvp, I'm assuming you're somewhere in the northern hemisphere. In that case you have your heat on since it's cold outside. The dry air makes static electricity build-up relatively easy. Rubber shoes and wooden floors will only make it worse. Use a humidifier or stop wearing rubber shoes near the NUC.
Don't allow it to build-up. By having very dry air and wearing rubber shoes on wooden floor you allow yourself to build-up static electricity. Then when you touch anything that is grounded - your metal window or the NUC you discharge what you have build-up.