I'd understand if we'd turned up planning to stay for hours. But DS and I were in the area, and it would have seemed rude not to pop in for 10 minutes, IYSWIM. So we did, and they welcomed us in, and we chatted for 10 minutes in the garden - and then they said, "We prefer visitors to call before they pop in." They were washing up when we arrived.
Anyhow, I feel like a dog with its tail between its legs. I don't mind (in fact, rather like) friends descending on us for brief, impromptu visits - but that's me, I suppose. Were we being unreasonable to just pop in for a few minutes, unannounced?