"evidence a very poorly performing employee was reading articles on the web most of the day"
Sounds like probable cause for a warrant. Not that employers needs a warrant to use their own property, but to assure employees that they aren't being watched unless there's legitimate suspicion.
A warrant??? I'm not aware of any country where you can/need to get a warrant to "spy" on your employees. In some countries that would be illegal. If said employee manages to do what others do while mostly surfing the web that's great. If they aren't delivering then by all means fire them (ideally with some sort of process to actually let them fix it). I don't see what the spying buys other than the mistrust of employees, the news getting out, and people not wanting to work for you.
