On my aircraft I observed a decrease in fuel pressure on two separate occasions while in the cruise at 8500ft. A subsequent data download showed a commensurate decrease in fuel flow. There was no noticeable change to engine sound. I had tight parameters set on the fuel pressure at the time, otherwise it may have gone completely unnoticed.
During troubleshooting we removed the fuel servo finger filter (fuel injected engines) and found it contained debris. It appears that it was clean before test flying commenced and may have trapped debris from the electric fuel pump running in. The electric fuel pump is downstream of other fuel filters.
This filter was not itemized in the maintenance schedule, so has now been added.
Two subsequent periodic inspections of the same filter showed lower debris quantities and then running completely clean.
Interestingly an internet search of this item showed a number of instances where it had been omitted from inspection - one referring to it as "the one that gets ignored".