In other words, if all five of your senses are in working order, a sneak attack is just about the only thing you don't have to worry about. So at the end of the film, with the three surviving protagonists standing in the middle of Central Park (the one place in Manhattan that lacks giant monster-hiding buildings) it's certainly shocking when the monster suddenly appears and (spoiler alert) eats the wacky sidekick, but it also raises some questions. Namely: When did the monster stop making a big explodey scene everywhere it went and start tip-toeing up behind people like a 100-foot-tall ninja? And how did Hollywood know you wouldn't ask that question while watching the film?
We're calling it Peekaboo Ending because it relies on the same ass-backwards logic that makes infants squeal with delight when someone hides and unhides their face. When we're born, we believe that things stop existing if we can't see them. To an infant's mushy, half formed brain, peekaboo looks like their mom is blinking in and out of existence with a stupid look on her face. Roughly translated, those squeals mean, "Holy shit, mom's a wizard."
Or occasionally, "Gaaahhhhh! Kill it, kill it, oh my God somebody kill it."