A: This is an excellent question. Playing Hide and Seek is age-appropriate and fun for your daughter. "Sneakiness" is not. You say, "She knows exactly what she's doing and why." I assume you mean attention-seeking.
My first very important suggestion......Put all medication bottles, keys, and other dangerous and necessary items out of her reach. Safety is first.
Then, tell your little girl that when there are two people playing it has to be fun for both people. It's not fun for Mommy when you hide Mommy's things so it's not a game. But it is fun when we play Hide and Seek in our backyard (only in the enclosed section, so she doesn't run out of the gated space). Then take her outside into your enclosed backyard and take turns hiding. Be sure when it's your turn to not hide for long before popping out from behind the bushes and saying, "Here I am!" I want her to learn a new shorter time span in which hiding is acceptable.
Also, create a mini-behavioral approach. Do your best to not make too big a deal when she next hides something. If she she gets a rise out of you it reinforces her negative behavior. Rather smile positively and say, "Show Mommy how you can give back Mommy's keys now so I can give you a sticker." If she drags her heels and doesn't return your keys immediately then she gets no sticker. If her sneaky behavior persists you may want to consult your pediatrician to get a referral to a child development/behavior specialist. This could be a sign of anger in your little girl. Good luck!