Amusingly, I actually lost a house key on the lower section of the track leading up to South Head a few months ago, but went back on a bike the next day and miraculously spotted it thanks to a semi-luminous key fob.
You don't say exactly where you found it, but if it was on one of the bridleway sections and you can be bothered, it might be worth mentioning it on some of the mountain biking forums.
