The picture shown above was shown off by Bungie September 9th, 2009, at PAX. This was during the time period ODST was getting ready to launch and they were doing some Q&A with guests and the media. Bungie stated, in this video here, that this image was of some kind of importance and they wanted people who were watching online and the people who were guests at the event to tweet to Marty taking guesses at exactly what the image meant and what QTA XII / VI meant. As time went on, discussions about the image circulated, theories spread, and eventually people forgot about the infamous QTA sequence.
That is, until it was spotted in Halo: Reach.
I feel as if this sequence is on the Marines for us to notice and remember it from ODST. I think Bungie wants us to go back to ODST and try and figure out once and for all what QTA means and why it's of importance in the ODST campaign. Only time will tell, and maybe one day Bungie will reveal what exactly QTA stands for and why it's everywhere.