Have you checked if the VM monitoring was enabled on the DRS cluster by any chance.? which could power on the VM.
Thank you for the suggestion. I have it disabled on all clusters and did check HA and DRS settings when initially investigating.
I never did find root cause for this. We are going through maintenance upgrades but I wanted to include the version levels when this behavior was witnessed:
vCenter 5.0.0 Build 913577
ESXi 5.0.0 Build 1254542