Answer by Professional Japanese Teacher
I don’t know.
I don’t know
It is better to use wakarimasen in the above scenario.
Wakarimasen means “you do not understand” or “you can’t give an answer”. Shirimasen means “you have no knowledge of something”.
Therefore, in the above scenario, since you have no knowledge of the whereabouts of your boss and you can’t give an answer, you can use either wakarimasen or shirimasen.
However, it may seem a little cold to use shirimasen. So if you know both answers are okay, it’s better to use wakarimasen.
Let’s look at another scenario. If someone asked you whether you know the phone number of another colleague and you don’t know, you can reply shirimasen since you do not have the knowledge of the phone number.
You can’t reply wakarimasen, because it sounds weird to say “you don’t understand” or “you can’t give an answer”.