Velvety, sit down and close your eyes, breathe calmly and repeat to yourself : I will find that jewellery box. You can use this prayer to St. Anthony of Padua that I use:
St. Anthony, St. Anthony
Please come down
My precious jewellery box is lost
And can't be found
Dear St. Anthony, I pray
Bring it back, without delay.
My Jewellery box's lost and can't be found
Please, St. Anthony, look around.
(memorize it, shorten it if you want, and repeat it)
(In French I use this):
Saint Antoine de Padoue,
Vous qui retrouvez tout,
Aidez-moi à retrouver
La /le________ que j'ai égaré.
It works for me every time! It must relax my brain in a certain fashion so that suddenly, I get a flash in my head, Look there! And most of the time, there it is. Or I find it shortly after.
If you had a house sitter you probably stuck it somewhere where they would not find it either, but it must have been somewhere unusual. Your brain forgot about it for either 2 reasons:
1) It's either too well hidden and your subconscious is asleep, knowing no one -including you! - will find it;
2) It was placed somewhere where there was a risk to be thrown away if not recovered immediately, and your subconscious is protecting you from the truth because it would hurt too much. Like in the garbage, with food or in the laundry.
But 2 would be less likely if you had a sitter. I hope this helps you.