Make your way throught the Koh Kret market place. You'll need to walk over a bridge to cross a canal.
Don't be in too much of a hurry to cross the bridge. Look around! Don't be afraid to speak with the local vendors. They are very nice, but they pledged they would not give away the secret cache location. You must find it yourself! You might be able to reach it from the bridge, or you can go along the small path beneath the bridge and then you're bound to find it. It is a black film canister with a magnet (so it must be stuck to metal).