It's the same in Denmark and it feels right to me. It's not like they are doing detective work (per default). If you have a concealed source of income, I'd expect them not to know about it. Of course, you'd still be breaking the law by not reporting it and it could turn up if they notice something that doesn't add up somewhere.

I assume this would still be a crime, and you'd be held responsible for lying on the past forms in discovered. The only difference is, this is optimized for making the common case easier - the case of typical income of regular, honest individuals.

Not really. Everyone is aware of what is automatically reported to the tax authorities.

Until you're selected for a random audit and quickly get charged with tax fraud.

They only know what they sent you. So an audit wouldn't turn up new information just force you to prove existing information.

Do note that it the responsibility of the employer to report who is employed, so if anything is missing, then it will generally be the company that is the one in the wrong. Especially as the company also pay employer fees for employees, so it would be a flagrant case for the tax agency anyhow.