What was Lucius looking for when he found the sealed-up paper?

On the night in question, Brutus cannot sleep.  He instructs Lucius, his servant, to light a candle in his study.  Clearly, Brutus' earlier conversation with Cassius regarding the plotted assassination of Caesar is weighing heavily on his mind.

Dutifully, Lucius finds a candle and goes in search of a flint to light it.  It is while he searches for the flint that Lucius finds the sealed-up paper.  While Lucius then heads for bed, Brutus reads the paper.  In it, an unknown Roman exhorts Brutus to "wake up" and to see what is happening in Rome.  Obviously, this is in reference to Caesar's recent actions.  We know that the letter is a plant, but Brutus does not, and its impassioned pleas drive him to join the assassination plot.