"Have you spent much time collecting bottles and had them appear in the same spot for several days in a row regularly?" - Yes. All bottles took me around 4 months (but I must confess that I didn't check it EVERY day). I had a friend who was exping on Roshamuul back then, he was looking for bottle "stones". It didn't appear for like a month of daily checks, but once it did, it lasted there for 3 days straight.