Thanks for the replies.
@kelstarrising: You are right - I rigorously Collect, Process and Organize stuff, but I skip the Weekly more often than I'd like. This indeed might be a weak link.
@TesTeq: I have insignificant Outlook reminders thanks to our wonderful company mailing lists. Sometimes they send a bulk e-mail that contains a meeting request, which then gets copied into my calendar automatically. This usually happens during the evening/night (while it's day in the USA), so sometimes I have random new appointments on my calendar when I wake up.
@vbampton: Glad that I'm not alone with the experience and that it's solvable.
What do you mean by brain dumps - do you have scheduled time periods when you go through your brain and empty it? My understanding was that one should jot down everything right when it pops up, so brain dumps seem redundant, since I should already have everything written down. Or did I get this part of GTD wrong?