Something new has shown up on your desk. It's nice but you're busy and it's not that important to you so you don't acknowledge its presence. Eventually, someone mentions it to you in the hopes of gaining some recognition for its delivery, assuming that you haven't noticed it yet. But you have noticed it and you mention this fact to their shock and surprise. Now, in this situation, it's better to say that you didn't notice the item in question until that moment, for trying to explain that you had noticed it earlier but didn't say anything is much more complicated and your co-workers likely won't believe your explanation. Because these kinds of things really matter to them and they would have said something had they been in your position. If someone had left a nice poem about the autumn or a delicious snack on my desk I certainly would have said something.