Those lights weren't the signal or else they'd be too late to indicate that Helena and Demetrius are distracted, also those lights being the signal would have indicated that Sirleck knew about the aberration nuke spell, but Pandora said that it was a spell that no mortal could get, and now that I think of it, might be considered the Ultimate Bane from which Pandora's descendants get their affinity from.
No, the original signal was to tell the vampires to attack and we never saw what that signal was, Sirleck might have just assumed that once the attack was under way then Elliot would be cleared for zapping. And it's likely he was risking it by assuming, but again maybe Helena and Demetrius let him and we're about to find out soon where they've been this whole time. Heck maybe there's been a miscommunication about which Immortals were spotted at the mall, if say Scarf Guy reported back to Sirleck that he's spotted 2 Immortals at the mall, Sirleck would assume it'd be the Immortals that he needed to be distracted, but in reality it was Pandora and Zeus that were seen, not Helena and Demetrius.