What are common causes? I have 181,500 km on my 2007.

I'm currently getting 16.5 L/100km (14.25 mpg). It was 13.5 L/100km (17.42 mpg) when I bought it at 157,000 km.
What is everyone else getting?
Hasn't thrown any codes.

Steadily declined over past 3 months from 13.5 L/100 km to 18.5 L/100km and then back to 16.5 L/100km.

I've maintained it meticulously.

All season Tires replaced 2000 km ago. Alignment done at the same time. - All fluids kept upto date using only Genuine Honda fluids. - Synthetic Oil changes per maintenance minder. - Spark plugs are 50,000 km old. - New battery last year. - 2 Transmission pressure switches replaced. - ECU reflashed last year.
Valve adjustment performed 13,000 km ago. Gasket was replaced.
Filters replaced.

 

Looks like you've covered all the obvious culprits. Is it hot where you live? A/C can drop your fuel economy by 5-15%, all other things equal (I personally have seen mine drop from ~25mpg to ~22mpg since summer hit...yeah, helps to be FWD and a highway driver). Also, seasonal fuel blend changes can affect your fuel economy. Higher ethanol content = lower mpg.

One thing I noticed - when I reverted back to stock from an ECU tune (decided I would just keep this as a plain daily), I actually gained over 2mpg on average. If you have the actual Flash Pro, consider reverting back to stock for a few tanks and see if there is a difference.

 

Might be too obvious to have not been mentioned, but have you adjusted your driving style? Staying out of the boost will increase your mileage significantly.