Just to revive this post. I don't know if anyone has this cover but to get to the hidden menu now when you tap Calibration setting for 5 times it asking for a password to go further and that password is unknown.
But here is the app to do the hardware calibration in case the ST16S goes nuts. Exit the flight mode app and go in pad mode and on the app's screen is an app called Flight Control Mode Test. Click on that and be careful what you play with.
Wisest words ever spoken. Too many threads here by guys that wanted to tweak this or that and end up bricking it. Unless you are having an issue, just fly it. I have not calibrated the accelerometer or gyro since December 2018. I last did my compass in January. I move all over California going literally from end to end and have never had an issue or need to re-calibrate anything.