Good
rule of thumb
as to was it a muff or catch & fumble:
If it had been a forward pass, would you have ruled it a catch ??
If yes, then it's a catch & fumble, kick's over, it is now open season on moose & footballs.
If no, then it's still a kick, K can recover but not advance and a kick is a kick, is a kick, is a kick....until to reaches R's end zone where it becomes a touchback.
IF unsure let play go, beanbag spot of K's possession and confer with crew for opinions. It's much easier to take the play back to the spot of K's recovery then to administer an inadvertent whistle.