Put it just before the reversal in an edit, then follow the turn with a Vine Boom to underline the change of direction. For a second version, open with Dun Dun Dun, let the warning land, and finish with the Vine Boom once the person is heading the other way.
Über diesen Sound
This very short effect lasts under a second and delivers its turn-back idea in one quick hit, with no build-up and nothing to wait through. It feels abrupt and urgent, like a warning flashed at exactly the wrong moment, and it works because the message lands before anyone has time to overthink it.