What are immigration officers allowed to ask me when I return home to the EU?

(This question is sparked by this recent one where the immigration officer asked an EU citizen where his flight departed which ultimately resulted in a two hour delay.)

I am an EU citizen of a Schengen country. I have both a passport and my country’s national ID card on me and I am allowed to legally enter the Schengen area on either. I have read in the corresponding legislation that upon entry the document may be inspected but it must be rapid. I was under the impression that there is no need for questions (indeed, I can’t remember being asked much, only a ‘welcome back’).

I wonder which questions the immigration officer is actually allowed to ask me if he has no reason to suspect my documents aren’t in order. Considering this is Europe, there should be legislation highlighting at least the general boundaries.

Clarification sparked by comments: this is specifically about arriving by air at an airport where customs and immigration are obviously separated and only about the immigration part.