The PIC24F RETURN instruction
There is some information in the 24F Programmer's Reference Manual, which is always a good place to go if you need to understand individual assembly language instructions.

On page 48 we find the following table, which is a slightly more useful version than the one in the datasheet:



On page 11 there is a small hint at the exception processing mechanism:


  2.1.1.6 EXCEPTION PROCESSING
  
  The 16-bit MCU and DSC devices have a vectored exception scheme with support for up to 8 
  sources of non-maskable traps and up to 246 interrupt sources. In both families, each interrupt source can be assigned to one of seven priority levels


So it seems if there is a non-masked trap or interrupt waiting it will only take 2 cycles for RETURN to finish and begin to service the relevant code.  If the return is going to "normal" code then it will take 3 cycles.