What kind of ADS-B identification code is FFxxxx, where xxxx are numbers i.e. FF0024?
I am going to assume you use Flightaware's variety of dump1090 as a decoder. 
When that receives a Mode A code, it publishes it as if it were a 24 bit address. The first two hexadecimal digits are filled with 'FF'. I have no idea why this may be a good plan.

In mode_ac.c, lines 368-370:

// Fudge an ICAO address based on Mode A (remove the Ident bit)
// Use an upper address byte of FF, since this is ICAO unallocated
mm->addr = 0x00FF0000 | (ModeA &amp; 0x0000FF7F);

These may be drones but most likely they are just simple aircraft replying to Mode A/C interrogations. 

If you are in the USA, FF7777 is a field test transponder (Range &amp; Azimuth Beacon Monitor, RABM) , usually located not far from a radar to verify the correct working of the radar.