If this gunk is on the floor of the boot (trunk?) and is matte grey in colour (my car is silver), then it is OK: my car has that, too. I think it is just the sealant used to rust proof the car. The fact that it is there might be reassuring: if the rear had been shunted the boot floor might not have this sealant.
Generally a car that has been seriously rear-ended will have ugly joins where the rear chassis members have been replaced. It is normally quite obvious and looks like very thick glue has been applied with a 3 inch paint brush! It is also usually painted over the same colour as the body finish.
Believe me, I know the paranoia that strikes when you buy a used car. I know the manager of our local bodyshop reasonably well--this place is Porsche approved and can work miracles. Last week I popped in to see how my dad's 996 was coming along (low speed spin while driving in snow, bumper and taillight damage only, btw). I was tempted to ask him to look over my C, as I know he can spot these things, but I thought: don't spoil the car, I like it, I'm happy with it, the last thing I want to know is that it has been bumped!
Scratches on the bumber can be parking scars; a slight mis-alignment does not mean the car did not come like that; and so on...