So your saying pfsense without any dns is reaching out to a specific IP? So the IP must be hard coded into pfsense to check for X?
I don't think so to be honest, hard coding IPs is horrible coding!
Lets see these logs, or the IP that its reaching out to.. And we can prob figure out what is going on.. But I would be very surprised if the pfsense dev's hardcoded an IP into anything they are running. Best would also be these sniffs you took.
You have no packages installed?
You sure its just not the ping to the gateway of pfsense wan? That would be reaching out to an IP without dns to resolve it.. You do know that pfsense even if you turn off unbound, will try and grab dns from dhcp on its wan. And then would attempt to use that for dns..
Also how are you sure its not something on the lan side trying to get to X?
What about NTP? If pfsense at any time had dns, it would of resolved some IPs in the ntp.pool and be trying to set time with those, etc.
TL;DR going to need way more info to try and help you figure out what your seeing.
Also, I have a few pfsense vms I could fire up and try and duplicate what your doing/seeing..