I'm in NL and often use this website to check for day-ahead pricing: https://energie.anwb.nl/actuele-tarieven

It's very frequent indeed. I noticed that it's mostly during sunny days in late spring / summer / early fall (solar). Much more rare is randomly at night (stormy days).

Solar is still pretty small in national grid charts, but I suspect that residential solar just reduces demand from the grid and doesn't show up as production.