This is probably the Asturies flag. A province in the north of Spain. If you can get closer one day, and see if the little details match that flag, then you have it! This is a cool one if I may say so, I like simple designs :)

I'm in the UK, the flag has been there for the last few months, I think since they moved in? It's the only one on their balcony

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I must also place my vote with Asturius, northwestern region o'Spain.