Flight Plans between Schengen Areas
When touring Italy in June this year I had difficulty in making almost any flight without filing a flight plan despite following the guidance in the AIC and carrying an ELT on board and declaring this before departure.
When departing Italy for Austria (via Brenner) and leaving Austria for France (overflying Germany) I filed flight plans on both occassions and was expected to do so. What was NOT required was to inform Customs of my departure or arrival.
Hope this helps.
Lom.