Your boss would just have to put a keylogger or use something like Ammyy in silent mode to see what you're doing (I assume Tor usage is visible in the firewall

Your boss would just have to put a keylogger or use something like Ammyy in silent mode to see what you're doing (I assume Tor usage is visible in the firewall logs hence making you suspect, but I really have no idea). If "they" have physical access to a machine, you shouldn't trust it.