IMHO there is no specific log dedicated to CAL assignments/recalls.
The Events Log (See C:\ProgramData\QlikTech\QlikviewServer) however has information about CAL assignments, both for Document CALs, Named User CALs as well as for automatically assigned CALs.
Look for lines that contain:
100 Notice CAL allocation:...
700 Information CAL allocation: CAL has been automatically assigned to...