Dell Optiplex 755 no display

My boss recently picked up a lot of 8 Dell Optiplex 755 PCs.  (Ultra small form factor model)
Only thing we know about these machines is that they were working a few months prior, and at that time, the drives were wiped.

The fans, hard drive, CD drive, etc all spin up when you press the power button, but there's no video output.  Tried a DVI screen (that was working with an Optiplex previously) and several VGA monitors through an adapter.  Every display goes into power saving mode, as if nothing is connected to it.

Tore one down the mainboard, nothing looks missing or out of order, all the capacitors are fine as well.  Also tried different power supplies.
Checked the diagnostic lights, they cycle between a few patterns before settling on 1 2 3 = GREEN 4 = OFF.  I looked up this code, but it's a generic "check drive connections" error.  The machine also makes two short beeps when attempting to start.  This beep code is not in the list.

What has me stumped is that all 8 machines do the same exact thing.  All I can think of is some fluke hardware failure, where all the mainboards or power supplies died in storage?

Any ideas?