It may not even have any significance, for all we know it could be a red herring! Left there for the suspect have us suspect it's someone else when really we should be suspecting them, but leaving us to suspect someone else because it couldn't obviously be them for making it so obvious, but... to distract us from something else.

[ Rapture, on the other hand, does not seem the least bit confused at the statement she just made. ]

For all we know, it may not even be an 'S'. It could be a crudely drawn 5 or some other kind of symbol.