diff --git a/cmd/testwrapper/flakytest/flakytest.go b/cmd/testwrapper/flakytest/flakytest.go
index 475a843ec..8b9dcd980 100644
--- a/cmd/testwrapper/flakytest/flakytest.go
+++ b/cmd/testwrapper/flakytest/flakytest.go
@@ -19,7 +19,8 @@ import (
 const FlakyTestLogMessage = "flakytest: this is a known flaky test"
 
 // FlakeAttemptEnv is an environment variable that is set by cmd/testwrapper
-// when a flaky test is retried. It contains the attempt number, starting at 1.
+// when a flaky test is being (re)tried. It contains the attempt number,
+// starting at 1.
 const FlakeAttemptEnv = "TS_TESTWRAPPER_ATTEMPT"
 
 var issueRegexp = regexp.MustCompile(`\Ahttps://github\.com/tailscale/[a-zA-Z0-9_.-]+/issues/\d+\z`)
@@ -33,7 +34,11 @@ func Mark(t testing.TB, issue string) {
 	if !issueRegexp.MatchString(issue) {
 		t.Fatalf("bad issue format: %q", issue)
 	}
-
-	fmt.Fprintln(os.Stderr, FlakyTestLogMessage) // sentinel value for testwrapper
+	if _, ok := os.LookupEnv(FlakeAttemptEnv); ok {
+		// We're being run under cmd/testwrapper so send our sentinel message
+		// to stderr. (We avoid doing this when the env is absent to avoid
+		// spamming people running tests without the wrapper)
+		fmt.Fprintln(os.Stderr, FlakyTestLogMessage)
+	}
 	t.Logf("flakytest: issue tracking this flaky test: %s", issue)
 }