It's confusing but clearly significant because the LAPD get another message from the killer containing the same initials but this time, apparently, in the middle of a sentence.
What that sentence is we can't tell but it looks like the killer is taunting police about finding him and saying, "...I hid myself". The placement of the 'BD' looks odd and there are a number of contexts you could put the final few words into to create different messages. See what you can come up with on that one.
The trailer finishes with Phelps' boss (who, for some reason, has quickly become our favourite character) getting angry and shouting, "If ever there was a time for extracting a confession this was it!" which points to the game's multiple routes, the chance for successes and failures throughout and maybe even towards the LAPD's willingness to "extract" a confession, which sounds like one of those "greater good" rule-bending exercises.
We've taken a load of evidence (that's right, evidence. Play along) from the trailer so that you can look for more clues or just marvel at how good L.A Noire is looking.