Continuity mistake: When they give Garber the pistol to take, it has actually changed when he pulls it out of the bag, it's an entirely different pistol.
Continuity mistake: The money car gets hit by an ambulance on its way to the drop and rolls over. The light bar on top breaks off. It reappears when the car falls off the bridge. (00:59:10)
Continuity mistake: When Ryder first walks into view over the Internet feed from the laptop, he is wearing a greyish T-shirt (most likely short-sleeved as well). In the next shot, still on the feed, he's wearing a black leather jacket, the same one we see him wear since the beginning. (01:01:20)
Continuity mistake: When Garber hands the hostage negotiator the page with his notes, the way Ryder is written changes. (00:24:40)
Answer: The hijackers did notice it. There's a scene where they notice it: look at the girl on the screen and then laugh. They get kicks from toying with people, which is why they didn't smash the laptop and just let the guy keep using it.
You're probably right.