Does this circuit detect frequency or magnetic field?
With those three BC547 transistors arranged as they are, that circuit will probably do a pretty good imitation of an electrometer.
A simple electrometer looks like this:

From the above linked web page.
It reacts to electric fields.  It can &quot;see&quot; batteries that are just standing there doing nothing.
They can also detect static charges, like an electrostatically charged wool sweater or blanket - or cat.
Being made of bipolar junction transistors (BJT) rather than a junction gate field effect transistor (JFET,) yours probabaly won't be as sensitive as the circuit above made from a JFET.
Your circuit is reacting to the electric fields, not specifically to AC.