I'm wracking my brain for reliable ways for an electronic device to detect when/if the parcel it is sealed inside is opened.
The device will be sealed inside the package and activated via Bluetooth by the sender and then queried again by the receiver to confirm there was no tampering before taking delivery.
At this stage the enclosing parcel could be a box, envelope, or wrapping paper - the only restriction that it completely encloses the contents along with the sensor and possibly packaging material - but that may include further restrictions if I can't come up with an infallible detection method.
My first thought was a simple ambient light sensor - but the tamperer would just need to open the package in a darkroom. Another thought is to actively pulse an illumination source and then average the amount reflected back from the inside of volume from multiple directions (to account for the device shifting inside the package).
Any strategies come to mind? I'm hoping this may already be a solved problem somewhere.