You have a fuel leak...a small one (hopefully) I suggest you pour about 5 litres of fuel in the tank, seal the fuel lid tight and park the vehicle in the sun for a couple of hours. For best results, raise the hood to expose the tank to direct sun and as a result high temperature and formation of pressure inside the tank. The first step in determining the location would be to move the vehicle from where she has been parked a while and look for a whitish spot where the fuel might have dripped and evaporated. Working backwards it should give you the location of the leak.
if that does not work then you would need to get under the vehicle to check all the exposed joints in the fuel line. Hopefully, you'll find one that is moist. If there is none than you would need to remove the fuel tank.
From what i saw when the Type 181 was towed to Khalid Sahib's workshop, there was some serious rust issues. Its highly unlikely that the tank remained safe from it. Check the underside of the tank for the whitish spot where fuel may have seeped and evaporated.
If there is none and the fuel line is still running on the inside of the frame tunnel then you would probably need to run a new one end to end, along the tunnel inside of the vehicle. Hopefully that would cure the problem. If all else fails ..blame PTI and Imran Khan. Not that it would cure disappearing of the fuel but because it is fashionable these days.
