not sure about ur case, but i've seen my fren's old proton iswara having minor petrol leak (gas like vapour) from the pipe connecting the fuel tank into the engine b4.
the car didn't explode on the road don't worry.... but only noticed when mechanic told him after he went for check up. it wasn't a gradual drop in fuel consumption but more of a sudden drop but he didn't understand why oso until the checkup.
changing that rubber pipe of course cost a bomb la.
mechanic says its rare, but says old car under d sun all the time may make it easier for rubber to leak.
Fuel consumption for Proton Iswara, Is that my car's fuel consumption normal