They had a warrant. They don't need to launch a stealth mission from the 50th floor. The front door works fine. They could have broken into the office to plant something during the fire. lots of plausible reasons the security would get disabled during a serious fire event.
Its not always obvious which crumbs are meant to dig and which are meant to be proven later so we don't really have an option but to try to dig everything.
Stormy's lawyer had the disc(and showed them where it was). Easier target than Cohen's lawyer. They might have been looking for something, but it wasn't the disc. More likely is to plant evidence. Raid Monday morning before the planted material could be removed.