It's almost always seen with a detonator rigged up to it (usually like
this one as seen in MGS4) but it doesn't actually contain one by default.
Incidentally if the police knock on my door asking why I'm looking up information on C4 and remote detonators, I'll tell them it was your idea. :P
EDIT: This is interesting:
http://www.inertproducts.com/inc/sdetail/734/5360
It looks like fake charges could be use actual M112 sleeves from production overruns.