It is a secret. Now that you have all the other items, there's a certain place you should revisit. I'm not telling which one, though. ;)
I found, thanks. And thanks for your game! (Sorry if you spammed by my replies, i many times misclicked and delete messages).
I've looked everywhere twice and then some, but an't find the last trophy I need! Is it possible you could give a small hint or release a walkthrough? Thank you in advance!