Using a GoPro with a Lens Cap on seems to be a common occurrence as I recently found out on our FB page when I told everyone about the amazing flight I had just done only to realise I had the Cap on the whole time. I was pretty frustrated because I missed one of those amazing Sunsets.
Mrs BMSweb came up with the idea of using a Smiley Face Sticker on the cap. They are Yellow and really stand out on a Black Cap. So a big thank you from me to my Amazing Wife :) Hope this tip saves you too.