Throw In

Question regarding throw in.... Team "A" kicks the ball out of bounds - ball bouncers off the bench and ends up back in the field of play - throw in awarded to Team B. Field of play is a multi use, many line, turf field....

Player on team "B" taking the throw in picks up the ball, within the field of play - never "leaves the field of play" and throws the ball in....

What's the correct call on this? I called it as an "improper throw in" and awarded a throw into Team "A", but since the player never put himself outside the field of play is this correct?

The throw in was not properly taken as the player had both feet over the touchline and on the field of play.  There is no requirement that the player must have left the field of play and come back onto the field of play.  Since the throw in was inmproperly taken by Team B, the throw in would then be taken by opponent Team A

On fields that are multi-use with many lines, the referee should be aware of which line is the touchline and when throw-ins are taken remind the players if possible.