You would think ESXi's web interface would display a warning that the system is in Maintenance Mode just like it does when SSH is enabled. It took a while to figure out why all the VMs were in an untouchable state: no power operations, no edits, can only Register and Unregister VMs.
Well, is the maintenance mode icon over the host not good enough? If not, I'm sure you could write an alarm for that.
I don't think a lot of people would like this, I know I wouldn't. I see events related to this in our monitoring tools, plus we schedule the vcheck reports which also report if any are in maintaintence mode
http://www.virtu-al.net/vcheck-pluginsheaders/vcheck/
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