Using the computer unnoticed?

I worked with a guy recently that enlightened me on something interesting and trying to find out how it was done. Basically he said one of the guys on our team was able to change something on the computer so that the mgmt couldn’t see where he went on the internet or what program he used on the computer. Since i watch an infrastructure now I would like to know how something like this is possible? Anyone seen any setting like this? Thanks..