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.
Continuity mistake: Zack and Cody's mom takes away the rats to Zack and Cody's bedroom. She goes behind the sofa, but in the next shot of the door, she's not to be seen anymore.
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)
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.