My parents had a blockage and it took a plumber to rod it out from the inspection hatch. He was quite blase about what he extracted - a bit like a doctor, he sees it all the time! But we were astonished, mum had passed the menopause, and my sister and I had left home, years earlier (and in any case hadn't flushed them down the loos for years before after we switched to the ultra-thin types), so where had they come from? The plumber said that he thought, from his experience, that they can get stuck in the pipes for a very long time, slowly gathering muck and other debris. Bleurgh.