No one likes to feel like we're not being respected by the people around us. We can feel that in a few different situations, but especially when people insist on borrowing items from us without our permission.
That's something that can happen a lot within a family. Siblings taking things from one another is not unheard of. My mom has 7 siblings, three of them who are women, and when they were young, they involuntarily shared closets. One of my mom's sisters would take her new pair of white shoes to go gardening in the dirt, use her perfume, and take her phone to talk with her own boyfriend. It was a time when most people had a single phone in the house, so privacy was scarce. My dad had given a personal phone to my mom as a gift when they were dating so they could talk freely, and that's the phone her sister used to take without permission. My mom hated that, of course, but there was little she could do to avoid it. My own brother used to enter my room and take things, and we never even shared clothes! That proves that no matter what era it is, or if they are all girls or all boys, sibling will go ahead and take things from us.
Until a certain point, that can be normal. After all, we share a house, a space, and a family with our siblings, so it's natural to feel like there's enough truts between us not to be necessary to ask for every single little thing. We can always set some boundaries with them, and even though it is annoying, it's not nearly as invasive as if someone outside of our family went through our private property.
Unfortunately, for this employee, it wasn't an annoying sibling taking stuff without asking; it was a coworker. That is even worse, because it's not like they live together or have built anything remotely similar to familiar trust, and yet, she thought it was a wonderful idea to go ahead and take important items from his office without so much as asking if it was okay.