Packet Filter (PF) firewall logging
Packet filter doesn't log on any file. It does log on a network interface named pflog0 (a special device in Unix-lang), see
man pf.conf

You turn on this interface with:
ifconfig pflog0 create

You suppress it with:
ifconfig pflog0 destroy

You can follow what does packet filter logs on this interface with
tcpdump or wireshark.
Ex. :
tcpdump -i pflog0

If you want to go any further with PF and without writing your own configuration files, I highly recommand to test Murus.