The scanners the RH-96 support, can tolerate being powered off with the ignition or another remote power switch of some sort.
The 536HP with the Siren App will be an entire different case. You can kind of remotely shut off the 536HP with a soft turn off, using the Sleep Timer Option, hopefully Uniden will had a soft turn off like a computer. Then as long as the power supply to the scanner is then interrupted and reapplied, the 536HP will power up in the on state. This feature will allow settings, recordings and other changes to be written to the SD card and hopefully reduce the possibility of corrupting the SD card.
I really hope Uniden is LISTENING to this and considering adding a remote, soft power switch from the Siren App and also allowing the Sleep Timer to be part of the soft power shut down, this along with a time delayed relay on the ignition feed, or better yet, configure the headlight switch sense wire as an ignition sense wire to allow the 536HP to be properly shut off and restart automatically when the vehicle is started back up. If the scanner is remotely mounted in the car where you cannot see it, you could care less about back light dimming on the display, use this additional wire as an ignition sense wire to properly shut down and restart the scanner. Firmware tweak and user adjustable options are all that is required as the hardware already has the sensing ability built into the platform.