No students show up to class -- what should an instructor do? Suppose an instructor shows up to teach a class and no students are present. Assuming that the instructor's institution has no formal policy on such matters, how should an instructor proceed in such a situation?

If it helps to narrow down the question a bit, assume that the enrollment is no larger than ~20 students or so.

I am in the US, but responses tailored for other physical locations are welcome, too.

The above question was inspired by the following related one: Is it okay for a professor to leave the classroom only 5 min past the class start if nobody has shown up?
At my former institution the rule of thumb was a lecturer waited 15 minutes and if no one turned up then the he/she was free to leave (this was common when the period was devoted to a problems class). However, even if one student out of the class turned up then we were expected to present the lecture. 

The only case where I didn't follow this was one time out of a class of approximately 30 students one solitary student turned up. This was rather extreme so I checked and found that the students were on a field trip that day (I hadn't been informed which was rather bad) and the student who had turned up often missed lectures and had, apparently, missed the lecture when the field trip was discussed.

Because of this, the majority of students had good reason to be away I decided not to take the class since, effectively, the students on the field trip would have been at a disadvantaged through no fault of their own.