The Thing About Heroes - S4-E10
Corrected entry: Stella realises the 333 stalker is Drew because the jigsaw puzzle piece she picked up from under his chair when she was looking for the missing piece from the puzzle with blue crosses was in fact from the puzzle with green crosses, which they had not yet received. Yet she goes back to the lab and places this piece in to the puzzle to complete it.
Correction: Stella went and got this puzzle piece after she found the first piece, from her car, and it was assembled (for them to realise it had a piece missing) but before they found the second puzzle. They then found the second puzzle, noting it also had a missing piece. However, Stella didn't figure out the missing piece they found at Drew's place came from the second puzzle until after Adam explained the x's thing, and she had another look at it, so she could then go place it in the right building, as they had assembled both puzzles together.