Some of the people who work with me are on vacation.
Some of the people who work with me are on relief for pregnancy issues.
Some of the people who work with me are working at a client for the day.
Some of the people who work with me are working from home today. [1]
Some of the people who work with me are on the next door. [2]
Conclusion: I'm all alone for the rest of the day, for the first time :-)
[1] - everyone has a weekly day for that.
[2] - our office is temporarily divided in two parts.