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.