Here is a USB drive that actually earns the word "stealth."
Phantom Drive, from Rootkit Labs, is an open-source USB that looks exactly like a cheap giveaway thumb drive. Plug it in and you see an 8GB drive with some boring files on it. Boring enough that nobody would question it.
The trick is the firmware. It runs on a CH569 microcontroller and intercepts every write at the USB bus level. The OS thinks it wrote your file, but the firmware caught the password string before anything hit the SD card. Create a text file with "password:xyz" in it and the hidden partition unlocks -- with AES-256 encryption on top.
No NAND chips onboard because of the component shortage, so it uses a microSD card for everything. That actually helps the disguise -- plenty of cheap drives are just SD card adapters inside.
"From the outside, the Phantom Drive, created by Rootkit Labs, appears like a boring old USB. It has that classic, cheap plastic-feeling look." -- Tom's Hardware
The firmware uses the microcontroller's unique hardware ID as a salt, runs it through PBKDF2-HMAC-SHA-256 for 100,000 iterations, and uses the result as the encryption key. That is solid.
The whole thing is on GitHub. Schematic, BOM, firmware, STL for the case -- everything. If you need plausible deniability for data on the move, this is the best open-source option I have seen in a while.
"From the outside, the Phantom Drive, created by Rootkit Labs, appears like a boring old USB. It has that classic, cheap plastic-feeling look." -- Tom's Hardware
"It's made for situations and places where you might be forced to reveal the contents of a storage device." -- Tom's Hardware
Source: https://www.tomshardware.com/pc-components/usb-flash-drives/open-source-stealth-usb-hides-an-encrypted-partition-behind-an-8gb-decoy-drive-phantom-drive-appears-as-a-regular-usb-stick-until-you-create-a-text-file-to-unlock-the-hidden-data