AC 50 Hz noise from an unplugged laptop?
Yes, inside a laptop and almost any other battery-powered electronic devices today there are power supply circuits that convert the battery voltage to other voltages.  But these typically operate at frequencies of tens or hundreds of kHz.  I'd bet a great deal of money that the laptop is not generating any 50 Hz noise.   If I had to guess based on my past experience with 60 Hz noise (here in the US), I'd guess that YOU are the source of the 50 Hz noise. Even if your left wrist is well grounded, your right hand will supply lots of noise to anything you touch (or even put your hand near).
To test this, try grounding the laptop not by touching it, but by some other means. And then try variations on how YOU are grounded.
Edit for additional info:
The power supplies I mentioned use high frequency oscillators, coils, and capacitors to step up and/or down the battery voltage, to get 5V, 3.3V, 1.5V, or whatever. They use very high frequencies to allow the coils to be very small. Note that these are all in use whether your laptop is plugged in or not.  For example, if you want to convert from 12V down to 5V, the popular way to do that is a DC-DC converter that uses intermediate
AC, rather than a simple linear regulator that drops the voltage by dissipating power (waste heat) in a transistor.
As for my claim that you are the source of the 50 Hz signal, I'm not suggesting that you are generating it, but rather that the human body is a surprisingly good antenna in that frequency range, and that your Faraday cage might not be quite as good as you hope.  (The human body does use signals in that frequency range, and that's the reason that a 50 Hz shock is more dangerous than a 60 Hz shock. But if you're seeing a solid 50 Hz signal, your body is receiving it, not generating it.)