I thought the same: after 7 days fast, I ate small 'lunch', consisting of RED grapefruit and 1 banana. 9 or 10 hours later I did the flush - so when I first saw them next day, I thought: yeah, grapefruit cells. But then I started to wonder whether I should check it more carefully. So I 'fished' one and... NO GRAPEFRUIT! The skin was very tough, and what was inside was no grapefruit juice; rather sort of a lymph
Apart from that - I don't think such delicate thing like a piece of grapefruit could pass whole GI tract without being damaged. You have to check your ones carefully, if they're flukes then you have much bigger problem then just gallstones!