Visible crew/equipment: At the beginning, you can see studio lights reflected on Zack's guitar during the song "Schools Out". (00:00:55)
It's a Mad, Mad, Mad Hotel - S1-E12
Visible crew/equipment: When Arwin tells about his super strange glue, there's a studio light reflected in his glasses.
Visible crew/equipment: In the episode "Hotel Inspector," when London comes to tell Maddie about something, she says, "Dogs playing poker," and you can see studio lights on her sunglasses.
Visible crew/equipment: When Cody tells his mother how much he hated Zack, you can see the reflection of a studio light on the chair he is sitting on. (00:08:45)
Visible crew/equipment: In the episode "Hotel Inspector", when the man with glasses comes in, you can see a light on his glasses that looks like a studio light. (00:18:30)
Grounded on the 23rd Floor - S1-E5
Visible crew/equipment: At the beginning of the episode 1:05 "Grounded on the 23rd Floor", when the big guy is falling over Mr. Moseby, you can see studio lights in his eyes when he yells "mamma". (00:00:40)
Visible crew/equipment: In the episode 1.09 "Band in Boston", Arwin comes in with his "Saxoblower." He puts on sunglasses and you can easily see studio lights reflected in them. (00:12:55)
The Prince & The Plunger - S1-E6
Visible crew/equipment: Zack and Cody go to Arwin the first time and get donuts, but when Arwin says it's going to be a steak next time you can see the studio lights reflected in his glasses. (00:09:15)
Grounded on the 23rd Floor - S1-E5
Visible crew/equipment: When Maddie gives Zack and Cody the "US People" magazine, you can see a studio light on it. (00:07:00)
Visible crew/equipment: Sometimes when they're at the pool and someone has sunglasses on, you can see the reflection of a studio light.
Grounded on the 23rd Floor - S1-E5
Visible crew/equipment: At the beginning of the episode 1x05 "Grounded on the 23rd Floor" after the big guy falls over Mr. Moseby and everybody looks at them, look at Zack and Cody's helmets: you can see a studio light. (00:00:50)
Grounded on the 23rd Floor - S1-E5
Visible crew/equipment: At the beginning of the episode 1x05 "Grounded on the 23rd Floor", when we hear the classic song, if you look closely at the door windows, you can see a studio light. (00:00:10)
Visible crew/equipment: When Chef Pablo shows them the greatest food, you can see the studio light and camera reflecting off the domes.
Visible crew/equipment: In London's suite, when Arwin opens the door to the porch where the hawk is nesting, the reflection of a crewmember wearing a white shirt is visible on the door's glass panes.
Visible crew/equipment: Episode 1x09 "Band in Boston", the reflection of studio lights are visible on the drums almost every time they are seen during this episode.
Visible crew/equipment: When Zack, Cody, Tapeworm and Max are singing "School's Out" on stage, you can see the shadow of the camera crane.
Visible crew/equipment: When Cody and Zack walk into London's suite and find Moseby there, crewmembers are reflected on the glass door behind Cody. First in the wideshot, when Moseby explains the plan to capture the bird and then in Cody's close-up.
Visible crew/equipment: Season 1, episode 24 'Crushed': When Esteban holds the tray with the dogs' food, the studio's numerous overhead set lights are reflected on the surface of the large, silver, dome lid.
Grounded on the 23rd Floor - S1-E5
Visible crew/equipment: When Maddie and the boys are crawling through the air vent, a boom mic can be seen on the right side of the screen.
Nurse Zack - S2-E29
Visible crew/equipment: After Patrick dumps the cake on Moseby, he wipes some away from his eyes. At the start of the next shot, when a crewmember's finger moves across the bottom of the screen it cues Patrick, who makes eye contact with the finger, to exclaim his line, "You did this on purpose!"
Answer: Keep in mind, Muriel wasn't very popular. So the writers/directors must have thought it'd be a good idea to let go of the character. No one seems to be complaining.