diff --git a/ntp_core.c b/ntp_core.c
index e64878b..4539cf5 100644
--- a/ntp_core.c
+++ b/ntp_core.c
@@ -309,6 +309,10 @@ NCR_GetInstance(NTP_Remote_Address *remote_addr, NTP_Source_Type type, SourcePar
   } else {
     result->do_auth = 1;
     result->auth_key_id = params->authkey;
+    if (!KEY_KeyKnown(result->auth_key_id)) {
+      LOG(LOGS_WARN, LOGF_NtpCore, "Source %s added with unknown key %lu",
+          UTI_IPToString(&result->remote_addr.ip_addr), result->auth_key_id);
+    }
   }
 
   result->max_delay = params->max_delay;