Penetrating German airspace pre-war 1939

[adriangunn]

Weird question: I am wondering how difficult it would be for an aircraft to cross into and traverse German airspace at night, and return undetected and unmolested?

A more specific (hypothetical) scenario:

In July 1939, military aircraft, such as a British Armstrong Whitworth Whitley takes off from an airfield in France near the German/Swiss border and flies at night over German to occupied Bohemia, drops some parachutists, and returns to France. A circuitous route along the Swiss border and then across Austria before turning north is around 400 miles or so (each way). Given the lack of hostilities, and lack of radar is it likely such a flight would be detected?

Regards,

Adrian