I've seen a video (now removed) where someone was drawing a bunch of blue lines (all separate, not making any contact with another). These lines were drawn on top of other lines, of various colors. Nothing was selected.
And then suddenly all the blue lines (and only the blue ones) were selected.
As no bounding box appeared around them, it meant they were not part of an Object Drawing or Symbol, which would explain the ease of selection.
How was it done?