PHL-SFO #969 on 2/21. I was upgraded 5 days out upon purchase of a B fare as a Plat. Boarding was a bit hectic due to the gate being one of the terribly designed ones located directly behind a bunch of support columns. During this I noticed there were two golds in front of me that had coach seats. So boarding finishes and I'm not really paying attention as I was seated with a co-worker. After we push back I realize that 4A and 2D are empty. I find it hard to believe there were no preferreds on the UG standby list, especially on a elite heavy route like PHL-SFO and given the fact that I saw two golds. So what happened here? Lazy gate agent? I would PISSED if I didn't get upgraded and the plane left with open FC seats on a long transcon. I did overhear a gold complaining to the FA about halfway through the flight when he probably noticed there was no one in those seats.