There are two hidden menus you can access on the 2018 Crosstrek and likely the Impreza. I am not sure about other models. I confirmed the below techniques I discovered by paying for a subscription to STIS and reading through the documents.
Access this menu by going to ACC mode (by turning key or pressing push button once with foot off the brake), holding HOME button while pressing TUNE button twice. Wait a second and release HOME button. This should bring you to the Factory menu where you can check some status and adjust a few backup camerasettings.
Access this menu by going to ACC mode (by turning key or pressing push button once with foot off the brake), holding HOME button while pressing TUNE button 6 times. Wait a second and release HOME button. If you press TUNE too slowly you'll get stopped at the factory menu after 2 presses (like i did once in the video below), so you need to get the timing right. This should bring you to the Dealer menu where you can check some status and access the diagnostics recorder. Other than that I don't think it is too exciting.
The Diagnostic Recorder in the Dealer settings menu allows you to download a diagnostic file to a USB driver. I plugged in a USB drive to the port in the center console, went to the dealer settings menu, turned off diagnostic recording by pressing "Recording ON/OFF", then pressed "Download REC data". Took out the USB stick and plugged it into my laptop. It contains a text file with only the following:
09-11-2017 07:58:13 0xA24D 0x00 SET
09-11-2017 07:58:13 0xA24D 0x00 CLEAR
Not too exciting, but interesting that it can just be downloaded to a USB stick. Might come in handy down the road if we can figure out how to decode it.
So, there you have it. Please share if you find anything else interesting!