It's what Symmetra used to say when she had put her shields on all allies, now I think it's an alternate line for when she uses her shield generator ultimate. I didn't even pick it to be a trick question; I was just looking for something that sounded in-character for her without being as obvious as "teleporter online."
Thanks! There are one or two that are unintentionally tricky, like the defensive matrix one for Symmetra, but I tried to choose a lot of lines you hear frequently while playing.