diff --git a/includes/libs/rdbms/ChronologyProtector.php b/includes/libs/rdbms/ChronologyProtector.php
index 315af53cde0..32318e1834e 100644
--- a/includes/libs/rdbms/ChronologyProtector.php
+++ b/includes/libs/rdbms/ChronologyProtector.php
@@ -163,6 +163,26 @@ class ChronologyProtector implements LoggerAwareInterface {
 	/** @var float|null */
 	private $wallClockOverride;
 
+	/**
+	 * Whether we are assuming a semi-deterministic clientId.
+	 *
+	 * This is set to true on the majority of requests where a clientId wasn't set
+	 * in a cookie or query param, i.e. when there were no recent writes in this
+	 * browsing session. This is a temporary flag to determine whether and why
+	 * a client might receive a clientId but then not send it back to the server
+	 * on subsequent requests (i.e. due to cross-domain browser restrictions that
+	 * we may have not known about as part of Multi-DC prep in T91820, or due to
+	 * bot frameworks that may be ignoring cookies).
+	 *
+	 * We then use this flag to log a warning in lazyStartup() if there were in
+	 * fact stored positions found under the assumed clientId.
+	 *
+	 * See also: <https://phabricator.wikimedia.org/T314434>
+	 *
+	 * @var bool
+	 */
+	private $hasImplicitClientId = false;
+
 	/** Seconds to store position write index cookies (safely less than POSITION_STORE_TTL) */
 	public const POSITION_COOKIE_TTL = 10;
 	/** Seconds to store replication positions */
@@ -195,6 +215,7 @@ class ChronologyProtector implements LoggerAwareInterface {
 		if ( isset( $client['clientId'] ) ) {
 			$this->clientId = $client['clientId'];
 		} else {
+			$this->hasImplicitClientId = true;
 			$this->clientId = ( $secret != '' )
 				? hash_hmac( 'md5', $client['ip'] . "\n" . $client['agent'], $secret )
 				: md5( $client['ip'] . "\n" . $client['agent'] );
@@ -462,6 +483,12 @@ class ChronologyProtector implements LoggerAwareInterface {
 				] + $this->clientLogInfo );
 			}
 		}
+
+		if ( $indexReached && $this->hasImplicitClientId ) {
+				$this->logger->warning( 'found position data under a presumed clientId (T314434)', [
+					'indexReached' => $indexReached
+				] + $this->clientLogInfo );
+		}
 	}
 
 	/**