Brysing raised a good point, is it a sign or is it just the human brain trying to make sense of things? Both explanations are possible.
It's like when people keep seeing the same numbers- is it because they are signs or simply because they have ascribed meamimg to them and their unconscious mind picks up on them more and alerts the conscious to them?
Someone may have used the feather for a bookmark- I have used odder things myself.
I have often used a feather as a book mark. Dont assume every unsual thing you encounter in life is a "sign". I think Freud put it best...sometimes a cigar is just a cigar.
To be honest a sign is only a sign when you observe it as a sign at the moment it occurs. If it strikes you suddenly with intuitive knowledge (or previous obtained knowledge of said "omen") regardless of what you find on Google, books, etc afterward, THEN it is a true sign.
Signs should never need "figuring out". The only time a sign should be reflected on intently is when it is repeating. If you keep finding black feathers and this is completely out of the ordinary for you and you are entirely perplexed, then I would say that some form of divination should be followed up on the subject in order to further understand the message.
Otherwise, take it as you received it at the moment of receiving it, and merely "keep it in mind" as you live your life normally.
If it is indeed a sign and you were correct, then you will see it fulfilled in the near future.
I would ask your mentor what the sign meant if he/she saw it as a sign. Perhaps it was meant for him/her indirectly instead of for you directly. Sometimes messages get pushed through people more open to them to those that are ignoring them for themselves.
I'm thinking this could be a sign from Odin which has been seen with a combination of Ravens and in some cases Crows. This could be a sign to expect something in a Meditation. In some cases I have even heard of this being a streak of good luck.
