Revealing mistake: When Penny is launched out of the truck, she is supposed to be screaming, but it is apparent that she is smiling and laughing during the shot. (01:14:05)
Revealing mistake: When they freeze time towards the end, we see a boy jump into a pool and then the splash is in mid air. If you look closely at one point you can see him move a little underwater.
Revealing mistake: Before the warehouse scene, Gadget and Gadgetmobile are talking and Gadget's hat illuminates with hearts. Look at the strap: after it illuminates it turns to a smaller cloth substance. It is let out that there's a computer in the strap used to make the hearts illuminate on the strap.
Answer: Probably just a common trope seen in movies to indicate something is on or working. It adds a hint of in-movie realism. While it's shown that the claw can be operated manually or through will, it isn't implied the light does anything or tells us anything.