Actually, not really. I mean.... technically you need it to find the sigils. You certainly need it to passively "sense" them. However, once you know where they are you can reveal a sigil with human JOAT detect (I haven't tested it with true 0 detect skill such as on a gargl or a filthy elf) as long as you target the exact tile. Easy to do if you've marked the sigil spots on UOCart or UOAM like you should. Not ideal to steal sigils competitively but it does work. I've probably stolen 100 or so this way.