Participating countries are allowed to check your identity documents at a Schengen border in order to confirm that you are entitled to cross it without any other steps. They're even allowed to record that you crossed, using the ID to track who you were (and Poland does), though they aren't allowed to record that on a passport or similar travel document.

Having confirmed you're an EU citizen who isn't subject to an arrest warrant you're entitled to cross the border and there are no other checks permitted. They aren't entitled to ask you to explain where you're going or why or anything like that.

There's a similar internal border not far from me at the local airport. There's a painted line and it has a nice sign which explains that they don't actually have any officials present at the border, but if you don't have documentation allowing you to cross or if you've arrived with products that are prohibited you can phone them, they'll send somebody over in a few hours to interrogate and if necessary arrest you. There's a phone provided, it looks pretty dusty. I guess it's possible that people decide not to call?

> from EU countries

Not really.