Is it peculiar for a department not to have any of its own PhD graduates as professors? I have been doing research into graduate programs and have noticed that some departments have a representation of their own PhD graduates as professors while others have an absence.

For example, a department I have looked at in a top university in Canada had a faculty completely composed of PhD graduates from top American schools, but none from its own.

I can see this as a signal of either two things -


That the department may not want to be seen favouring its own candidates.
That the department may not have as much faith in its own graduates as that from other schools.


I understand that this may differ from field to field.

But in general, what is the majority view-point on a department that has no representation of its own graduates on its faculty?
It's common in computer science for a university to have few if any of its own graduates as faculty, and should not be taken as a bad sign by itself. This low representation is for two main reasons:


The best five or ten universities in the United States produce a disproportionately large number of strong academic candidates. In practice universities that have research departments at all (let alone the exceptionally strong ones) tend to be filled with professors with Ph.D.s from the elite few.
Going somewhere else leads to cross-fertilization of academic thought. Presumably after 5+ years of graduate school one has spent a lot of time learning from the expertise available in their department. Going somewhere else allows a professor to transfer her knowledge to a different group of researchers, and to get fresh insight from them in return.