Half of fundamental frequency in Fourier transform I understand why there are peaks in integer multiples of the fundamental in the Fourier transform. But why is there often a peak in half the fundamental?

I am testing for the guitar specifically. I am doing an STFT on an guitar audio monophonic signal and then get the 4 strongest frequencies in each note. I always get the fundamental in those 4 and of course some multiples of it but also half of it.
When I play A3 (220Hz) in my guitar, the fifth string which is A2 (110Hz) also vibrates a bit: it is what is called Sympathetic resonance. Besides other non-linear effects, this could be the case.
This phenomenon also occurs sometimes when recording bat calls, you can  observe resonance at half the fundamental frequency.

Here, the fundamental frequency is at 65 kHz (upper orange tag) and there is resonance, probably generated by the microphone, at half that frequency (lower orange tag).