It must have been a flashback of Jackson triggering the device.

Master Zack's recording clearly stated that once the device was pressed, it would explode when time ran out, but Jackson focused solely on comforting her despite imminent danger to himself.

"I will handle any matters regarding Zeke Shaw from now on, so please don't contact or see him and his family again," Jackson said between gentle kisses to her lips.

Still lost in recollection, Serenity was jolted back by a sting on her neck.