Can Teemo's Legend ability (Swift Scout) bounce a facedown card from the board to hand?

No, this is an illegal play. Swift Scout's ability states "Put a Teemo unit you own into your hand from your Champion Zone or the board." "'Unit,' 'gear,' and 'rune' refer to objects on the Board unless specified otherwise," and another rule states "'Facedown card' refers to a card in a Facedown Zone unless specified otherwise." A facedown card is not a "unit" - it has no card type and is a different game object category. Therefore, even though the controller knows what the facedown card is (since it's Private Information they can look at), they cannot target it with Swift Scout's ability because the ability specifically requires "a Teemo unit" and a facedown card does not satisfy that requirement.