diff --git a/object/service.proto b/object/service.proto
index 4116de6..d31422a 100644
--- a/object/service.proto
+++ b/object/service.proto
@@ -350,6 +350,10 @@ message SearchRequest {
     // * $Object:PHY \
     //   Returns only objects physically stored in the system. This filter is
     //   activated if the `key` exists, disregarding the value and matcher type.
+    //
+    // Note: using filters with a key with prefix `$Object:` and match type
+    // `NOT_PRESENT `is not recommended since this is not a cross-version approach.
+    // Behavior when processing this kind of filters is undefined.
     message Filter {
       // Match type to use
       MatchType match_type = 1;