Just run past the window when the guard's shadow isn't visible on the shades.
The missing security guard is in the clown store. Talk to him to get his time card, then go back to the museum. Once you get to the museum, go to the security office, and then click on last night's security video.
You don't see him at night when the painting is stolen, but the next day he is in the Clown Store.
Talk to the museum security guard, who is in the clown shop. He will give you his timecard, which shows when he was away from the gallery. Go to the security office and look at the video from the night before, and you should get a photo of the thief.
You can find out by looking at the security videos in the museum Security Room. You will need the time card from the night security guard, who is at the Clown Store.
There is security lighting in the gallery itself, notably above the painting "The Scream".
When entering the museum at night, you slip past the security office and then avoid the lasers in the statue room. When you reach the painting, jump onto it. This may not seem wise, since you immediately get caught...but it is the next step in the quest.
After you take the lie detector test, go to the Clown Store and get the timecard from the night security guard in the museum. Take it to the security office at the museum, and check the security footage for the previous night. You can get a photo of the theft, and you see that the thief was the guy who "warned you" about the theft. Show his photo Downtown to see if you can locate him.
Wait until her shadow moves away from the Security Room window, then run across.
To pass the Security Room, wait until the guard's shadow is no longer on the window.
When the painting is stolen, get the timecard from the night security guard (he's in the Clown Store). Then you can see the theft on the videos in the Security Room of the museum.
get it at night
Counterfeit Island