I noticed there was nothing on the web about it also. So it really sounds like a made-up term, but it has a ring to it that could lead to a repackaging and perhaps a book. As Jake's first suggestion is what I would say is the closest to reason.
IMHO: A signature is a set of parameters and actions which can be associated, due to uniqueness, to a specific type of defect or failure. By being aware of signatures, a really sharp tester could conceivably predict a defect before it is observed. Some of the signature could be developed by use of a code usage analyzing tool which can pinpoint the redundancy of use for a specific module within the application, thus highlighting the over use of that module which could result in a defect in the future. Also I would presume that part of the signature could be made up of the proficiency and coding methodology of a specific developer. ie. A developer who is lax on synchronization.
So we could "categorize" the defect and then do a root cause analysis either by developer or category (Pareto diagrams are good for this) to identify if there is a tendency for one person to be weak in and are or for the team to have a specific weakness.
Rich - want to collaborate on an expensive 500 page book for this?