It's probably a pathfinding issue. One of the Sims 3's biggest problems is anti-climatic moments due to pathfinding.
It can really ruin the moment when my newly married couple has the house all alone. They wake up, and snuggle for a little while on the bed before waking up, and enjoying a hot meal together. They spend the day working out in their garden, stopping a few times to splash each other with water. After they're finished working, they head out to their outdoor garden baths for a relaxing soak under the tree's shade. At around 8pm, they head in for a romantic dinner. My female sim prepares lobster thermidor, and my male sim goes to the basement to fetch some nectar. After their dinner, they gaze into each other's eyes ravenously.
My male sim grabs my female sim, one thing leads to another, and they're ready to head to the bedroom for some woohoo.
It takes 2 hours for my sims to get in bed.
This is where imagination really comes into play. The reason the mummy didn't go after your sim is because they're so light and dainty that he didn't hear her. Or, as you said, because you're sim was just too pretty to eat.
Either that or wait for a patch.