wmic bios get serialnumber returns blank after virus scan

Under an administrative account if "wmic bios get serialnumber" is run the output is

SerialNumber

*blank line*

This happens on 4 different Dell workstations each of a different make and model and there is no correlation with a particular version of Windows

Our virus scans entail:

Run Microsoft Security Essentials

Run Ccleaner

Clean any junk files in %appdata% and %programdata%

Run Malware Bytes

The asset tag shows in the bios and we have tried reflashing the bios to no avail.

Any ideas on how to get the asset tag to show again?