Or he heard it in the background at the grocery store, Restaraunt, or mall. I could see a kid hearing something a parent was too busy to hear in a public place. If he did hear a conversation in Russian and it stuck in his head long enough to repeat it correctly later, yeah, I'd be fishing around for someone who might understand too.
I told the hubby what I'm typing right now because he's sitting here watching tv and he said yeah, our three yr old just heard a song in the car and we came inside and he was singing it. I know I've been at the store and he hears a baby crying across the store, faintly, that I didn't notice. I know because he says something.
So, yeah. It's more likely that you saw what you thought you saw happen and it's related to giftedness.