Half of fundamental frequency in Fourier transform
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).