gas milage has dropped from average 40.4 to 33.5
Over how many tankfuls?
What else has changed (tire inflation, wheels and tires, driving circumstances and speeds, terrain, loads, roof rack load, etc.)?
If it was because of an engine malfunction, there would probably be a check engine light on and maybe flashing.
now that the warmer weather is here, using the a/c might effect your mileage a little.
The nine year old battery may be failing.
Start with the basics: tire pressure, dragging brake (hot wheel), engine thermostat, wheel alignment.
You may be experiencing the built-in shutdown of the battery. Read this:
I did the fix on my 2011 MKZ hybrid, mileage went back to normal.
+1 to Texases comment
This “feature” has been pre-programmed into Ford’s hybrids, but luckily it can be overcome, as Texases has done.