Logs below come out when bridge goes away. The scenario was bringing up stack, creating networks, external router, vms and floating ips. Then killing the stack session.

The errors below come out every ten seconds so it looks like there is a corner case where the arp responder should be stopped. When the stack was killed the br-ex is also removed which is likely why the bridge cannot be found. The arp responder code should see that the bridge was removed and then remove this thread looking for the gw.

2016-01-10 18:00:33,149 | ERROR | pool-45-thread-1 | GatewayMacResolverService | 285 - org.opendaylight.ovsdb.openstack.net-virt-providers - 1.2.1.SNAPSHOT | MAC address for gateway 192.168.56.1 can not be resolved, because external bridge 116666960027975 is not connected to controller.
2016-01-10 18:00:43,149 | ERROR | pool-45-thread-1 | GatewayMacResolverService | 285 - org.opendaylight.ovsdb.openstack.net-virt-providers - 1.2.1.SNAPSHOT | MAC address for gateway 192.168.56.1 can not be resolved, because external bridge 116666960027975 is not connected to controller.
2016-01-10 18:00:53,149 | ERROR | pool-45-thread-1 | GatewayMacResolverService | 285 - org.opendaylight.ovsdb.openstack.net-virt-providers - 1.2.1.SNAPSHOT | MAC address for gateway 192.168.56.1 can not be resolved, because external bridge 116666960027975 is not connected to controller.
2016-01-10 18:01:03,149 | ERROR | pool-45-thread-1 | GatewayMacResolverService | 285 - org.opendaylight.ovsdb.openstack.net-virt-providers - 1.2.1.SNAPSHOT | MAC address for gateway 192.168.56.1 can not be resolved, because external bridge 116666960027975 is not connected to controller.