windows 2008 r2 - unexpected reboot

We have 3 servers which have rebooted unexpectedly all within 3 days of each other (each one approximately 24 hrs after the last one). In each case what we are seeing is absolutely no memory dump, crash dump, or notification in the Event logs giving any clue as to why the servers have rebooted. From the hardware side we have no visible issues via DSA and/or IMM. Monitoring of the servers shows no abnormalities toward cpu, memory or disk usage.

We're curious if there's anywhere else we might look short of engaging Microsoft themselves to see what condition might result in these scenarios.