Ground rules question

[freejoe69]

Situation: Batted ball lands fair down left-field line, hooks past outfield fence into out-of-play area. Outfielder does not signal with two hands raised, but rather chases ball into out-of-play area. Does the ball become dead by the simple fact that it rolled out of play, or rather when fielder signals that it has done so? As the third-base coach, in the absence of a signal, I had batter-runner continue around the bases. After a lengthy conference, the umpires returned him to second base. I contended that in pre-game, umpires had emphasized that on ball in that area, or ball rolling under outfield fence, play would continue in the absence of an outfielder signal. Interpretations and citations, please? Thanks in advance.