How does a Wizard becoming 'untraceable'?

[Jen Reese]

No: HPFGUIDX 104940

According to the FAQ on JKR's website, a witch or wizard can become 
untraceable just like a building can be unplottable. JKR said this 
in response to the question of why the MOM didn't send an owl to 
Sirius and follow it.

Any ideas? It must tie in with Dumbledore in PS, when he said there 
are other ways to become invisible besides using an Invisibility 
cloak. 

Maybe there's nothing more to it, just another complex spell in the 
WW that we haven't heard about yet!

Jen