"local host is behind NAT, sending keep alives" even when it isn't

[Isabella Borgward]

Firewall with 2x WAN interfaces with public IPs. 1 LAN with private IP. Only routes to far end of tunnel are using WAN interfaces.
I see that "local host is behind NAT, sending keep alives" even though there is no NAT involved. How is the firewall determining that NAT is in use?