[patowalker]

Enlightened NAAs adapt their FPL rules to local conditions. There is a lot of flying between bordering countries, where filing a flight plan is an unnecessary nuisance that causes delays and adds nothing to safety. Just ask any pilot based near Schengen, where airfields in four different countries are a few minutes away. Safety is not enhanced if pilots opt not to file a FPL and simply go non-radio. I know, because I was based in Luxembourg for seven years.

The Belgian AIP has this

any flight across international borders. VFR flights remaining within the Schengen Area do not need a flight plan as far as the Belgian part of the Brussels FIR is concerned (for requirements applicable in other Schengen States, please consult the relevant AIP).

Unfortunately, so far, that only applies to flights to and from Germany.