When the Man, one of Lord Triassic's distant cousins, inquired about hiring C'Esther to dance at a party, and invited her to dinner to "discuss the project" at a fancy restaurant for Men and their cat companions, she put on a thick layer of happiness to tell her father about the opportunity. When the Man withdrew his interest that morning, she said nothing to her father.
She eats dinner at Delectable C'mestables as planned, bringing one of the maids so that the log records a couple. She finishes quickly and returns to her father's block, loitering by the business entrance, covering her mind with tired boredom.
After half an hour, a caterer arrives. One of her father's mice holds the door for delivery cart and she prowls through, barely covering her mind at all - she's been here many times before with her father and can easily conjure up a copy of her remembered emotions: duty, wonder, attentive thought.
Getting into the secret laboratory requires walking through the non-secret laboratory, which is filled with night-shift technicians, but she has an excuse: a deliberately-ambiguous records request that the clerk served incorrectly that afternoon. Following up on it gives her a reason to be there, and the clerk will be glad of the opportunity to correct the error and pretend it never happened. (She feels bad about wasting his time - she'll send him some easy requests later to pad his numbers.)
And finally, with less than an hour left in her alibi, she curls up under a desk in the secret laboratory with the only copy of the stroon-eater development notes, trying to figure out how to inconspicuously sabotage the next round of experiments.
The fire alarm screams. She slams the notes into their hole, rolls back the carpet, drags the filing cabinet over it, steps away, turns back to check that the cabinet is perfectly lined up with its indentations in the carpet - she's feeling a bit nauseous and dizzy for some reason but it's fine, no one's going to remember her leaving while they evacuate - she stumbles, tries to catch herself on a chair, claws scrabbling - she passes out.
And dies a few minutes later.