I uploaded pictures from a class trip on Facebook back in 2009 and had no captions or names that suggested that the pictures were taken in California. My camera at the time was not capable of GPS. Facebook still managed to get the location right.
Like I said on Quora, we use the location field that you put into the album when you uploaded it. The album location isn't displayed in the caption for individual photos and isn't displayed that prominently on the album page, so you're probably just not seeing that you set it when it was uploaded in 2009.
It didn't work like that. It was a plain blank text field among a couple other blank text fields. No search was brought up upon data entry. Very innocuous; Perhaps that why you don't remember it.
Believe me, I would have remembered doing it. I wouldn't have cared about a location tagging message if I had entered the location manually. Not to mention I couldn't even remember the names of some of the cities where the photos were taken until Facebook suggested them to me.
Scary stuff.