A piece of tech that busts every camera in the building surely is a clear sign that your building is being compromised, no?
The only SF I've read that has surveillance cams in it is
A Scanner Darkly by PKD, where, of course, he comes up with a completely messed-up scenario. Of no help for your story, but one of my favourite books!
Regards to your issue. Disguise? Walking about as cleaner, say. And then acting 'normally': i.e. act like a cleaner, or act like someone working late. Scoping out where all the cameras are, would be I think important, as that tells you where in the building to be careful and where you can relax and fiddle with any equipment.
Also it'll depend on who or what is in control of all the feeds? Is it an ever watchful AI that can trigger a call to security on a hair-trigger...or is it a tired human night-guard at 4 in morning, who could also possibly be distracted. Like internet security it is mostly the human element that is at fault, not the tech.
More high tech thoughts...why not break into the 'feed' of the cameras and broadcast a video that just shows a loop of time that had no one on it, so that the whole system is effectively blind and you can do anything in front of the cameras. (See Ocean's Eleven
)