Ivee knew her way around Sophie and Seldon's kitchen just as well as around her own, which meant that she was able to pay attention to her cooking and not be mentally scrambling around to figure out where things were. Which meant that she didn't have to pay attention to anything beyond the food. Seldon knew better than to come begging before she called him, and Sophie was working, and not likely to come in without being called.
Which meant that Ivee was at the cooking unit, stirring the soup and humming quietly to herself, her back to the door, with no reason to think to listen for it.
The queen and Sophie couldn't have asked for a better set up.