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Question: 08 Passat - Car starting to have SIGNIFIANTLY high fuel consumption

08 Passat Komfort (2.0T) 41000 miles

About a month ago, noticed that my car's fuel gauge on the dashboard is dropping significantly (both the needle movement, and the available drive-range that appears on the computer system). 08 Passat is known as (according to Internet) having 22 mpg in-cities. Currently the mpg on computer system appears to be 21. However, as mentioned above, the needle movement and available drive-range is dropping crazy, like when I drive 4 miles, it consumes about 20 miles drive-range, and the needle drops about a whole tick (between the lines in fuel gauge). This is not what it used to be for the entire 2 years of owning this car. So three weeks ago, received wheel alignment. Went back to normal for a week, started to do it again, checked alignment and balance, and start plugs, but it did not improve. Went to VW dealership 2days ago, found out at 40,000miles, they provide fuel sensor and tank related service - they said this might make the fuel gauge looks like it is consuming more but actual gas is still there. After receiving them, it went back to normal. But from yesterday morning, it started the same problem again, went back to normal fuel-consumption for couple of hours, and later being problematic again.
Is this sensor problem? - meaning the actual gas will be there but just appears to be consuming a lot, or actual engine, etc related?

There is no check sign on on the dashboard. If there's something leaking or anything that's performance-related, it will probably show up on the dashboard, right?