Can happen if you remove animals from exhibits (for example if you remove all of them to sell some off) and people are looking at them when you do it. Also if someone if you are looking at an animal and it is boxed or escapes etc. Basically anywhere where a guest / keeper is looking at something and then it is removed from that container (for want of a better word).
My biggest bit of advice, do not over think staff or guest thoughts, it is the route to madness......