The log showing the offline scan run seems to be stored in a file below C:\Windows\Microsoft Antimalware\Support, using the naming scheme MPLog-.log (e.g. In previous versions of Windows: Select Threat history.
On the Virus & threat protection screen, do one of the following: Select Start, and then select Settings > Update & Security > Windows Security > Virus & threat protection. To see the Windows Defender Offline scan results: According to Microsoft, any threats detected by the offline scanner will show up in the Threat History (where the online scanner also records any viruses found):
