Again interesting item.

Our point of view is:

For some organisations it is possible that HR knows when people are gone.
Also it is possible that a department organize it.
So a lot of information about holidays can be very decentralized within
the organization.

Mostly users activate the Out of Office options in their email system.
So by getting those information from your mail server you know exactly:
which person aren't present and which person should be present.
But is this interesting?

Absolutly NOT.

An user account is personal. Possible mail must be forward automatically to
another person or your are using department/group mailing lists.
A policy within our organization is that the department organizes work in such
a way that there will be never a situation that people only can look out of
their window because a person is away.

For special tasks we have introduce functional user-accounts: no related to
a person and only with the required functionalities for those tasks. Mostly
to prevent an enormous amount of problems by moving users capabilities to
another and to prevent that tasks can't be done because the configurations
and authorizations aren't right.
Such special accounts we are monitoring for unexpected things.

Of course it can be possible in some situations, that an account must be
entered by another person. In those cases our organization uses a form, which
must be signed by persons with the power to it. Form contains username, reason
and period.
After verification of that form the network-administrator will change the password
and after specified period the account will be blocked.
By policy documents users know the rules.

btw: in an organization 'accounts payable' also only possible by a small group of persons.

btw: by surfing to unwanted sites on the internet people know they can be fired.
question: why should you risk your job, when internet is in many countries for
free or nearly free of charges?

Greetings.