In the Black Widow's lair, the female guard is on the third level and walks back and forth. Go down the path under the second level, then climb up above the guard on that level, until you can see the female guard. When she walks over and turns back to the right, jump up and walk carefully behind her, past the TV to the springy chair. Jump onto the chair and bounce up (and to the left!), to the key card door at the top.
The female "security guard" stays in the Security Room at the museum. The female guard who works for the Black Widow is on the third level of the lair, and walks back and forth to the left. When you climb past the first guard, jump up behind and carefully follow her until you reach the chair. Jump in the chair and bounce to the left.
Just run past the window when the guard's shadow isn't visible on the shades.
To pass the Security Room, wait until the guard's shadow is no longer on the window.
You must go on the Underground Tour before you can apply for the job at the museum.
The red-haired guard you meet at the Museum is at the Clown Store after the painting is stolen. The female guard is in the Security Room. Slip past her at night to enter the museum. The next day, get the timecard from the guy guard (at the Clown Store) and take it to the Security Room in the museum.
To get past the guards, go all the way to the left, climb above the boxes to avoid the male guard, and when you see the (female) guard on the third level, follow her and bounce up on the sofa to the locked door. (Use the key card to open the door)
Wait until her shadow moves away from the Security Room window, then run across.
There are no snowballs on Counterfeit Island. (see the related question)
Hid behind the statues and/or plants.It is too dark for the cameras to see you. The things to avoid are the female security guard in the office, and the lasers in the statue room. (Hiding behind the statues works.)
At the museum, you can't enter until you have taken the Underground Tunnel Tour. (see related question)
To get past the guards, go all the way to the left, climb above the boxes to avoid the male guard, and when you see the (female) guard on the third level, follow her and bounce up on the sofa to the locked door. (Use the key card to open the door)
There is no sign of him when you slip into the gallery at night, but you have to get past the female guard in the Security Room, and the lasers in the statue room. He is evidently still on duty after you are arrested, when The Scream is actually stolen. If you see him at the Clown Store, his timecard will let you determine what part of the videos to look on.