Messed around and forgot where I stored my Nightingale Armor.

yeah i came back to an old assassin save, began looking in all the player homes for the ancient assassin armor, finally found it on one of the initiates following me.

never that had happen to me but i feel your pain i would be pissed off if i could not find important idem

I think you can get another set by going back to the Nightingale hideout near Riften (I forget what it's called - Nightcaller Temple? Something like that anyway) and clicking on the stone again. It might be worth a try if you can't find your original set.

I'm too ocd about my stuff. All I have to do is look in specified chests in one of my many homes.

The worst for me was when I dropped my Savior's Hide armor in the empty firepit of my pre-upgraded Breezehome. I was in a hurry and I figured I'd find a more permanent place for it later. In the meantime, I upgraded the living room. The next time I entered Breezehome there was a fire in the pit and my Savior's Hide armor had vanished.

NPC's be having sticky fingers.

This is why my current character is not going to purchase several houses, like my first one. I was forever forgetting where I'd put stuff. And I stored and never found again my Ebony Mail Armour, which I loved.