DHT11 and DHT22 unused pin
On the DHT11 it's connected to pin 6 of the SOIC14 inside. So it's at least connected to something. But who knows what it's for.

It has a (internal) pull-up resistor to VCC of around 100kOhm on this pin.

I connected my logic analyzer to it, there is no signal, even while reading the sensor. Also connecting the arduino's signal wire to this (incorrect) pin does nothing.

Factory calibration or testing would be my guess too, but who knows.