Why does immigration check passports at some Schengen-only terminals?

Since the Czech Republic's probationary days as an EU state (we're talking the very early 2000's here), the Prague Airport has been the site of numerous pilot tests sponsored by the EU. As a result, the airport has developed a favourable infrastructure for pilot tests.

A pilot test can be anything from a variation on the procedures or more often the introduction of proposed equipment/technology.

Further, the end-user demographics vis-a-vis airport size make it a great locale for training, and the airport hosts border guard trainees from other member states (as does Poland and various other EU members).

However departing passengers are always checked by immigration, although they don't stamp passports. And I've seen the same configuration in other Schengen terminals, so I know it's not a Prague-only issue.

It's not necessarily always... A given pilot test can run anywhere from a month up to 24 months and during that time the eagle-eyed traveller would expect to see slight variations.

What's the rationale behind this? Note that I'm not asking about why airlines check IDs on Schengen flights, just why immigration is doing this.

It's seemingly against the Freedom of Movement Directive to do this, but there are carve outs for the purposes I have described. As you pass through the control point you may notice variations in their uniforms (green shirts, blue shirts, and white shirts) along with differing insignia on their epaulets indicating if there are 'guest' border guards. And a closer look at the equipment they are using will give an indication of a piece of technology that the EU is considering.

Finally, there are currently 7 member states who have used Article 23 of the Borders Code to implement temporary border controls inside of the EU so far this year. They are Belgium, Denmark, Norway, Sweden, France, Germany, and Austria. The Czech Republic is not on this list (at the moment) so you would not expect to see special measures in effect for that reason.

Adding: personal note, I'm one of those people when the border officials are doing something unusual who says "why are you people doing this?". I have never done it in Prague, but do it lots of times entering/exiting the UK (or Russia or the US or etc). Sometimes you get incredibly helpful info, and sometimes you don't.

Free circulation of citizens within an area doesn't mean that you won't get checked anytime. Countries immigration and police staff can perform routine checks anytime at airport, in the street or wherever they want.

You are telling that this happened even before the refugee situation. I still believe the controls have been reinforced recently within EU because of this.

There could be few reasons for doing those checks indeed :

• make statistics about people leaving the country
• catch people with invalid IDs
• catch people that could are being researched by police (maybe they won't take the risk to get controlled but you never know)
• show the population that the police is doing his job :)