Other mistake: While a sniper is awaiting permission to shoot, a rat crawls up his leg and bites him, resulting in an accidental discharge. A trained shooter, much less SWAT sniper, knows that you never place your finger on the trigger until you are prepared to shoot. Otherwise, you could have an accidental discharge.
Other mistake: When Regina first speaks into the mic, she doesn't press the button so they shouldn't be able to hear her. (00:11:50)
Other mistake: When they give Garber the pistol to take, the cop shows him how to operate the safety. He releases the safety and says it is safe, then puts it on safe and says it is ready to shoot.
Other mistake: In the scene where Garber is in the helicopter, the far away shot shows it as being almost sunset, when it is actually closer to 3:30. The sun is low on the horizon.
Other mistake: Following the scene where Phil Ramos is shot in the head by the sniper, you will notice him still breathing behind Ryder showing him in the background when Ryder gets on the radio.
Other mistake: When the man is saying the train will stop when it hits a red signal, the child immediately says that they're all green. From where this child is sitting with his mother, and with how short he is, there is no way he could have seen the signal lights. (01:51:00)
Other mistake: The young man's computer is discovered and then powers off because it was at 0% battery life. Just after the scene where Garber separates himself from the Ryder and his crew, the man reopens his computer, with it immediately reconnecting to the internet chat with his girlfriend. Obviously, there is no way that the computer would have powered back on without being recharged, let alone reconnecting to the internet and picking up at the exact place where it left off. (01:25:35)
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.