Sorry I misspoke, it's in DefaultTransport:Outgoing:send
From...
if (targetNetworkNumber != Address.LOCAL_NETWORK && targetNetworkNumber != Address.ALL_NETWORKS
                    && targetNetworkNumber != network.getLocalNetworkNumber()) {
                // Going to a specific remote network. Check if we know the router for it.
                linkService = networkRouters.get(targetNetworkNumber);
                if (linkService == null)
                    handleException(new BACnetException(
                            "Unable to find router to network " + address.getNetworkNumber().intValue()));
            }
to...
if (targetNetworkNumber != Address.LOCAL_NETWORK && targetNetworkNumber != Address.ALL_NETWORKS
                    && targetNetworkNumber != network.getLocalNetworkNumber()) {
                // Going to a specific remote network. Check if we know the router for it.
                linkService = networkRouters.get(targetNetworkNumber);
                if (linkService == null) {
                    handleException(new BACnetException(
                            "Unable to find router to network " + address.getNetworkNumber().intValue()));
                    return;
                }
            }