Newcomer here -
Labrat, postdoc and others have done a great job at educating everyone about the system in place in many university research/lab animal facilities - thank you.
I have worked in similar facilities for nearly a decade as grad student, postdoc and research scientist. Ours is a quite large, modern, new vivarium in the basement of a research tower. Just imagining the unimaginable - it is hard for me to believe that someone could do this in our facility during "business hours" without detection. As postdoc described, ours is arranged in suites, each containing 5-6 individual animal housing rooms and 1 or 2 procedure room. In our facility, each suite is shared by mutliple investigators and there are research personnel (i.e. grad students, postdocs, lab techs) and lab animal techs (i.e. those that work exclusively in the vivarium) going in and out all the time. All of these doors (to the suite, to the animal rooms and to the procedure rooms) have windows. The image one may get from the term "basement" is one of a dark and isolated environment, but these are modern, well-lit, fully staffed facilities.
Certainly, these facilities can be maze-like, and an investigator can have exclusive access/use to an animal room (i.e. unshared with other investigators). But, using our facility as a context, such an act would have to be meticulously planned, or completely spontaneous, if it occurred between 8am - 4pm. Such a bold, horrendous, senseless act.