Further research has been most fascinating.
Wawa:
Did you have a look at the pictures OP added to the original post.
Only just now, with my post just above.
Wawa:
I see some transistor added to a board with a threshold detector.
So no capacitive touch, and there might not be a big need for a ground plane.
The setup is just mains hum detection, and there might not be a lot of mains wiring near.
So I firstly (starting with lowest price of course 
) found this advertised as "Touch Sensor Capacitive Touch Switch Module DIY for Arduino Digital TTP223B Z4S3"
What is firstly fascinating is that it has a microphone attached and no part seems to resemble a TTP223B (it's an LM393).
Next candidate entitled "1Pc Metal Touch Sensor Module KY-036 Human Body Touch Sensor A MD JCAU".
Oh look! Another LM393. 
Some TO-92 device with a missing middle pin connected to terminals labelled "S" and "G". Wonder how this is supposed to work? Temperature sensor perhaps? Or a FET?
Final candidate, titled "1pcs KY-036 Metal Touch Sensor Module For Arduino AVR PIC". Tests for metal this time. 
Similar to, but not quite identical to the last. Also has the white blotch underneath. Almost certainly a LM393 in the shadow.
This one is in the OP's picture. Presumably is a transistor or FET, no instructions (there) as to how to use it or how it works, but OK, further research, it is a KY-036 module, the device is a Darlington.
So what are those other boards then?