Never heard of 'key in the door' syndrome but it does happen to me too at times - and I'm relieved to know it's a common occurrence.
At least it's easy to ask when you speak the lingo...
On one memorable holiday in France we stopped in a small town because I needed the loo. The public toilets were the hole-in-the-ground type, stinking and swarming with flies and made me retch - no way could I use them, desperate or not. Almost in tears and having searched elsewhere, I asked at a nearby hotel and was told brusquely 'NON!' even though they must have seen my distress. I finally found another hotel where I was treated kindly and shown beautifully clean toilets to use - I was so grateful I thanked the receptionist profusely in my rubbish French and explained I'd been turned away from another place.
I didn't find it easy to locate public loos in France - it's my main worry whenever we go abroad anywhere, but in Italy, for some reason, it was far easier and people seemed more accommodating.
Years ago, when I was a child, my mother bought me a copy of The Good Loo Guide. I loved that book.Henceforward we always graded public toilets in 'stars'. Wonder whether it's still in print?