I have an Arduino Uno and what it does is simply output an oscillating signal of 1 MHz on PIN11. I measured the signal using a oscilloscope and a probe with a ground clip. First, I connected the probe to a wire which is plugged into PIN11 and the clip to the wire connected to GND. The oscilloscope showed me the expected signal.
Then, I connected the probe to the GND wire and removed the ground clip from the probe while an open wire is plugged into PIN11. The oscilloscope showed me a weakly oscillating signal of 1 MHz. I extended the open wire which is connected to PIN11 and the signal got stronger. Then I removed the wire and the oscillating signal disappeared. My first guess was that the open wire is acting as an antenna.
I also tried to connect the ground clip to the GND while the probe stayed disconnected. Also, an open wire is plugged to PIN11. I expected a weak reversed oscillating signal but the signal was flat.
Now I am not sure again if the weakly oscillating signal is caused by the open wire which I thought it acts as an antenna.