From the observation that Jill made at 5 o'clock, we can infer that the assistant left work early.
This inference is based on two pieces of information: Jill saw her assistant's office light out and did not hear the sound of typing. The fact that the office light is out suggests that the assistant is not present in the office. Additionally, the absence of the sound of typing indicates that the assistant is not working at their desk.
While it is possible that the assistant could be on vacation or working in a different office, these options are less likely given the specific observation made by Jill. If the assistant were on vacation, it would be more reasonable to assume that their office light would be off for an extended period of time, rather than just at 5 o'clock. Similarly, if the assistant were working in a different office, it would be unusual for their office light to be out and for no sound of typing to be heard.
Therefore, based on the observation provided, the most logical inference is that the assistant left work early. This could be due to personal reasons, completing their tasks for the day, or any other reason that prompted them to leave before their normal working hours were over.