Mate exactly the same thing happened to me!!!
2 years ago when I bought my first boat (privately) I took it to a well know boat dealer on the beaches to fit a 2nd hand Yamaha I bought from them. The big day came to pick it up, was hitched up to the car ready to take off to the ramp, and my mate who was with me spotted it at the last minute. The bottom bung was clear, but the bung above that was behind a perspex type thing that the had the motor mounted over. Thank f&*k he noticed it, because being my first boat I would never have known.
The dealer ended up doing a dodgy and installed a new thread on the INSIDE of the boat, so to put the bung in you would have to move the fuel tank, get on your stomach and reach back behind to the transom and put it in that way. It leaked terribly, and was a massive hassle to insert and remove the bung. I copped it for a few months, then went back and made em fix it properly. They ended up just drilling out the perspex over the bung so I can fit it from the outside again.
PM me if the dealer was on the Northern Beaches, cause if he is the same bloke then it's worth reporting.