Where has Bob logged on? You can scan the security logs to get this information. SCCM sometimes is available to give you a list. But the list of people who have logged on is also available from the computer itself.
PCDomainLogonHistory.vbs creates a list of users who have logged onto a computer. It takes a list of names as input. If you have Excel installed, I use it for the file open dialog, and to resave the log in Excel. If not, you get a text file.