Generally a firing is decided the previous day or at least an hour before it happens. Discussions are made prior to the actual meeting where the firing occurs. IT is on standby. They either deactivate the AD account and related auth methods when the employee walks in the office to have the discussion. This is a well oiled machine, so that all parties know their parts. The meeting/discussion is solely a formality and by two minutes into it, theres no longer any access granted. Security shows up at the meeting to escort the employee out and collect their badge or keys. Maybe they let the employee walk by their desk to collect their stuff, maybe the employer ships it to them later, depends on the circumstances and office layout.