Leopards, animals accustomed to hiding themselves, definitely go through a process of slow adaptation when they first go on display and aren't likely to make an easy appearance.
When the red pandas first arrived, they often hid in caves or up in the trees, and Red Bean would get startled by a big rock.
Seeing so many people outside, Diana naturally didn't show herself but instead hid on a high tree trunk, her eyes covertly observing the visitors.
The lush leaves around her completely concealed her figure. She lowered her body, and her exposed black fur didn't stand out at all, blending like a shadow. The visitors' gazes wandered confusedly around the exhibit, sweeping over her many times without ever noticing her.
It might take a week or two, or even a month, for Diana to gradually let down her guard and engage in more activities outside.
Whether the visitors could see her or not was all down to luck and Diana's mood that day, whether she felt like making an appearance or not.