I work for a company that provides temporary housing for corporations and was looking for a source of a letter to tenants that may be left in their unit when we perform maintenance.

A simple:

Dear Tenant:

We needed to enter your unit today to perform necessary maintenance. We were there from 11:00AM to 1:00PM. The work done was carpet cleaning.

Thanks for your cooperation

Do the tenants sign something that gives the management/maintenance permission to enter when they are not there? Or is that a given, in this situation? If it is an understanding from the beginning, then go with what ScottGem stated. Something simple and easily understood.