Some of you may have some insight into this -- right now, I see on FlightAware, two Fedex aircraft which are essentially circling around seemingly aimlessly.  One is a DC-10 (MD-10) over Mobile, Alabama (Brookley Airport), and the other is a 757 doing lazy circles over southwest Georgia/north Florida, out of Cecil Field near Jacksonville, Florida.

I've heard Fedex sometimes has "reserve" aircraft airborne, ready to fly to a spot that has a lot of demand.  Do you think that is what these are?   Or are they doing checkride or maintenance ops?  It seems odd to me, that they are flying around Cecil and Brookley, as you would think they would be flying near higher population centers.  Links to flight paths below:

https://flightaware.com/live/flight/FDX9032

https://flightaware.com/live/flight/FDX9033

Both those airports have heavy maintenance facilities.

As a former Airline-FAA ATC guy (COA/UAL), I recall my FDX colleague describing exactly the case you mention on certain dates depending on business demand. Specially designated aircraft would enter holding to await possible re-dispatching to a specified station, or RTB (return-to-base) to MEM.