@Gertjan:
PC's and other devices could have 'static' DSN addresses set up, so they will contact for example "8.8.8.8", bypassing completely the local DNS authority (your pfSense).
That makes sense and explains those queries, thx!
@Gertjan:
Also : some devices, some software have DNS hard coded - you can't do anything about that, except blocking all outgoing DNS request, forcing the device to use pfSense, or have it shut up.
I do force all DNS requests to use pfsense only!