With intel from UA-cam User anubus6 via Ingvar on the HitmanForum.
Collect 4 "Bird's Eggs" scattered across the map (you don't need the mushroom). After taking out the targets, go to the roof of the Biker's Hideout and throw the Bird's Eggs into the frying pan of this white bird graffiti four times as it moves around the roof. The bike will emanate gas and the exit should be usable.
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