Source IP if no Self IP Defined

I have just taken over a F5 instance and I am trying to understand the routing. I have pool members which do not reside on the same networks as any of my self ips though somehow traffic is managing to make it to these pool members. I have a default route configured on the BIG-IP which I assume is being used though the BIG-IP seems to be using random self ips when looking at a tcp dump. There are no vlan groups. I now have to configure new services without self ips though I have no idea what the source ip may be to create a firewall rule from big-ip to pool member. Thoughts?