For context, the three main characters are visiting a museum and being sneaky by doing it while their parents are out. I know the gist of the line is "We're the heretics for taking advantage of our parents being out on work and [プラついてる] since noon like this?" In my research I keep finding references to plastic, which doesn't seem right, and I don't know of any good sources of slang or common sayings.
親が仕事で外出してるのをいいことに、こうして昼間からプラついてる私たちの方が異端ってわけか。