Dell Vostro 420 Power Always On

I have three Dell desktops connected to an Aten KVM:

No1 Dell Vostro 420
No2 Dell XPS 8700
No3 Dell Dimension 5000

This setup has worked faultlessly for a year or so, but the KVM has started behaving strangely. So I replaced the KVM but the replacement KVM has the same issues (as the one it replaced) so it's not the KVM.

After some investigations I have narrowed the issue down to the Vostro 420. The Vostro actually powers up the KVM via a USB connection. The problem lies with this USB power connection.

I have noticed, that after shutting down the Vostro (using Start->Turn Off Computer), the computer shuts down, all the fans turn off and the Vostro appears quiet, BUT power is still sent to the KVM (via the USB connection) indefinitely. I have measured this at the KVM end and it is about 3.3V, sufficient to keep the KVM lights on. This is very odd behaviour. The other two (good PCs) don't do this, power down means power down. But on the Vostro, power down means there is still some power coming from the USB connection. The only way to fully power down the Vostro is to pull out the mains lead from the back of the case. Then the lights on the KVM go off.

Is this normal behaviour? After shutting down a Vostro 420, does it maintain a low voltage on the USB connectors? Is the only way to fully power down a Vostro to pull the power lead out? Has anyone seen this behaviour?