Dark Mark question

[lisanicr]

No: HPFGUIDX 76355

I couldn't find this referenced anywhere but...
I'm re-reading GoF and in the scene where Karkaroff shows Snape that 
his Dark Mark is getting clearer...does that mean that the mark is 
normally, or relatively invisible when circumstances dictate? I mean, 
when LV is not around, does the mark disappear completely? Perhaps 
that's why it was such a job identifying true DE's after the downfall 
of LV?
Thanks!
Lisa