This has actually happened to me multiple times, so I can relate.
On weekdays, when my dad was at work, I would wait for her to run some errands. I always made sure that she would be gone for at least 30 minutes. I would then sneak down to the basement (where she usually hides my electronics), and search every shelf, cabinet, bag, and box until I found it. The last time I found it down there, it was at the bottom of a Ziploc bag with a few other devices and accessories. For good measure, I also swapped out my black New 3DS with a small black TV remote facing down, using it as a decoy so she would never know it was missing unless she looked closely.
Other times, (like most recently) she hides it in her locked bedroom. Occasionally, when she goes to the basement, she will leave the key (which is attached to a keychain with both the basement and bedroom keys on it) in the door. When this happens, I adjust the thermostat to get the heat or air conditioning running. The noise from that helps to mask the sound of me slowly removing the key from the door. I then unlock her bedroom door, then quietly return the key to the basement door before she notices it is missing. I can relock her bedroom door without the key, so I no longer need it. I then turn the heat or air conditioning off temporarily so that I can hear if she is coming upstairs. Until she comes upstairs, I search through her room until I find what I am looking for.
For the past few months, my New 3DS is safely in my room, and she has no idea.
My mom has also misplaced her keys a lot in recent years. One time when she misplaced them right before taking me to the gym, I slipped them in my gym bag (where I had also secretly stashed some money away), and after she dropped my off, I went to a store with a key copying machine and made some copies. I no longer have the copied key to her room, but I still have the copied basement keys.
If you still cannot find it, the other option is to buy a secret 3DS that she does not know about.
My parents have never read my GBAtemp posts, so I am not worried about posting this.
