In Harry Potter and the Prisoner of Azkaban (I think it was this one), Harry, at night time is seen in his room at night under his sheets doing the Lumos maxima charm, as he was underage, and in muggle society?
This scene was at the start of the movie, and Vernon entered the room two or three times.
Why didn't the ministry do something about it as Harry still had the tracer on him?
There are a couple of reasons why didn't the Ministry pick him up.
There are many rules in real life, but usually, authorities relax those rules in certain cases like when the deed is not that serious and let the accused go with just a little warning.
The same thing happened here. The Ministry is not gonna punish every kid who practices small spells outside of Hogwarts.
From the script for Prisoner of Azkaban,
HARRY: No thank you. Minister... I don't understand. I broke the law. Underage wizards aren't allowed to use magic at home --
CORNELIUS FUDGE: (dishing up a bowl) Oh, come now, Harry. The Ministry doesn't send people to Azkaban for blowing up their aunts! On the other hand... running away like that... given the state of things... very, very irresponsible.
Sirius Black escaped from Azkaban and it had something to do with Harry Potter. Therefore, it was more necessary to protect Harry than punish him. This is why the Ministry relaxed the rule for him.
This is one of those subjects that just does not make sense. The trace detects any magic performed near a minor wizard. The Improper Use of Magic Office cannot tell who is using it and is responsible for determining what usage consists of a violation. In the 2nd book, Harry gets accused of performing a hover charm that Dobby had done, however there is no accusation in regards to Dobby apparating in and out of the house. They obviously know Harry cannot apparate, yet they make no connection to the hover charm that someone else was on the premises. How many times were adult wizards at the Dursley's residence performing magic with no ministry warning. Hermione also admitted to successfully trying out "a few simple spells" just prior to her first year, no ministry warning. How about when Hagrid tried to turn Dudley into a pig? Neither of them are allowed to use magic, no warning. How about Riddle killing his father at 16? I imagine the response to that would have been more than a letter.
But if we really wanted to try and explain this...muggles call light coming from the end of a stick "Flashlight". They might just let that one slide.
