1998 Volvo S70 glt emissions failure
 

It is supposed to test the evap emissions system occasionally by pulling vacuum on it. I am not sure where all the measurements are made, I should study that. So anywhere gas fumes are controlled by the vent system, you could have a leak and the ECM could be correct in what it's telling you. Gas cap of course is the first place to look, and you probably already know that. The short rubber hose connecting directly to the fuel tank also seems to be a weak point. If it leaks, though, you usually smell gasoline. The parts of the system that are in the engine bay may have some rotten rubber.

I agree. There is a short section of hose under the RH rear tire that I've seen come loose or even rot away. Replace that and you should be just fine.