The fuel use in my S60T has 140,000 miles has recently gone up. The check engine light comes on periodically and I can watch the fuel gauge drop. My mechanic us unable to capture computer codes that indicate problems with electronic fuel modulator or fuel injectors.
It sounds like you might have dirty injectors, assuming your mechanic has checked the air filter and the fuel filter. For the injectors, it would not cost you much to try a bottle or two of fuel injector cleaner. Chevron’s Techron is often a recommended brand. It might require multiple applications or even a manual cleaning of the injectors if they are really clogged.
It could also be that your thermostat has reached the end of its life. Many newer thermostats break in the open position, which would keep your car from reaching its proper operating temperature.
Do the wheels spin freely when the car is up on jacks? A bad bearing or four could cause a loss in fuel economy.