It is perhaps the very key to our future existance. The 355th day of the year is December 21st. The Mayan calander ends on December 21, 2012. Coincidence? I think not.
When puzzled, or confused, it is used in place of such phrases as "Huh?" or "What?" Unlike grrr, "Grrr?" is not said in an agressive tone, rather a questioning, perplexed tone.
Person 1: (Mumbles something incoherent)
Person 2: Grrr?