With the exception of at large buildings in dense city centers, just about everywhere else, utilities enter a building at just some point on the back, out in the open. This includes utilities that feed alarms and security cameras.

While some places will have systems in place for situations where these outside connections have been severed, like independently operated cameras on an intranet, cellular data backup for alarms, etc., most places don’t, so successfully circumventing their security is just a matter of cutting all the cables on the back of their building at the same time, and then being gone before they notice