The idea of a "signature" by a killer comes, I believe, from Jouhn Douglas.

http://www.pentaone.com/hannibal/douglasarticle10.shtml

Linking Cases Together
Following a Killer's Signature
by John Douglas

Like everyone else, a killer learns from experience. If you don't catch him right away, he'll begin to develop his modus operandi, or MO, and probably get better at the crime.

Maybe he'll find a more efficient way to kill someone or a quicker way to abduct a woman from a car. He'll start showing more control over the crime.

The MO is basically the way the predator commits the crime -- if he uses a gun rather than a knife or lures prospective victims by putting his arm in a fake sling.

While the MO tells us something about how he did it, the signature gives us some insight into why.

Unique to the person

The signature is a ritual, something the subject does intentionally for emotional satisfaction -- something that isn't necessary to perpetuate the crime. Evidence of torture is a signature. Posing -- displaying the victims in a certain way -- is a signature. Signature is a way of linking cases.

Like a real signature, it's a personal detail that's unique to the individual.

Sometimes there's a fine line between an MO and a signature.

I've found that signature is a more reliable guide to the behavior of serial offenders than an MO. That's because the MO evolves, while the emotional reasoning that triggers the signature doesn't. The method a killer uses to get women into his van may change, but the fact that he always tortures them once they're inside stays the same.

One subject covered the faces of his victims. That was his signature. So you look for that in other cases in order to tie them together. A bomber used to spray black paint over the components inside his bombs. It wasn't necessary -- it didn't make the bombs any better. I don't know what it meant, but he did it anyway. He felt the need to do it.