Customs can certainly approach you any time right up to the gate. In 2010, my wife and I were approached by an agent and asked very directly if we had more than $10,000 in USD cash with us as we waited at the gate to board a nonstop flight from the US to Moscow. We answered truthfully "no" in a brief conversation and he wished us well almost immediately.

I don't think he suspected we were money launderers. My guess is that upon hearing both of us speak, he easily determined that we were not natives of Russia and thus not, for example, carrying but not declaring a substantial amount of cash intended for family back home.