Okay after further testing I have come up with a theory - not a good theory but a theory anyway.
Something was rattling around in my head about how often ESXi saves (backs up) configuration data to disk. My memory said every 10 minutes but after a little research this link suggests it varies: http://blogs.vmware.com/vsphere/2011/09/how-often-does-esxi-write-to-the-boot-disk.html
From here you can see that unless something more serious is happening in the ESXi configuration it will be once an hour (assuming a change has taken place). So what does this potentially have to do with your problem?
Well entering and exiting Maintenance Mode (MM) is a configuration change. If I enter MM on my test ESXi host then reboot the server normally, i.e. via the vSphere client when the host comes back online it will still be in MM. No worries here. Now if I exit the host from MM and then pull the power, after returning power to the host it entered MM on starting. This cycle will continue until I exit the host from MM and then save the configuration to disk either by rebooting the host via vSphere client (ignoring the error about not being in MM) or waiting an hour or so. (Full disclosure: Have not actually waited an hour to confirm this. Maybe later for fun or if I'm bored)
Consequently this also happens in reverse, i.e. if you put a host into MM then pull the power when it restarts it will not be in MM.
Now for the side track or why my test VM kept getting orphaned after a reset of the host. It was for the same reason as the above. I registered the VM then within a short period reset the host. vCenter knew about the VM but the host did not. Allowing the ESXi configuration to write to disk solved stopped this VM from being orphaned after a reset of the host.
Thoughts?