Can I represent physical rooms in Active Directory?

In addition to Arthurs answer, which has a slight weakness in utilizing an attribute which may already fill its given function in an organisation, I would submit two alternative possibilities:

1) Using the Exchange custom attributes, if the Exchange schema has been added to the AD. The original custom attributes are described here and the newer custom attributes are mentioned and linked to here.

2) Extending the schema with your own custom attributes.

Both of these options avoid using predefined AD attributes for a different task than intended.

As both methods are explicitly supported and encouraged by Microsoft, getting IT on board should reasonably be possible. You will not find it hard to google additional documentation when building your case.

While you specify "where a user is", Microsoft does recognize the need for identify where a computer object is location wise.

There is a location field in these objects that can be formatted and used for scripting queries, assigning printers, settings in GPOs, etc.

You can find more information here on SF about it: What is Location field in Active Directory used for?

Active Directory specifies an "office" field under the general profile information for any given user that could be used for this. You could also create a dummy security group for each room and do it that way. This would clutter AD, but reduce processing load for the warning system (since you're not iterating so much).