While folding laundry, I realized that if one sock in a pair was inside-out, the other usually would be as well. This thought was obvious after I had it, and also would have been obvious had someone asked me the question, but since it had never come up it was hidden to me. Is there a word for something that is obvious once you think about it, but unlikely to be thought about?
Not a single word, but a useful idiom.
Be (right) under your nose — Cambridge
to be in a place that you can clearly see
"I spent all morning looking for the book, and it was right under my nose the whole time."
"She shoved the letter under her boss's nose (= made certain he saw it)."
See more examples on TFD.