Some Uniden DMA/XT scanner special key codes work also in the UBC-3500XLT.
When powering on UBC-3500XLT press at the same time:

Firmware update mode: power+L/O+6

LCD pixel tests: power+0+3+7
                        power+0+1+9

Frequency bands test: power+0+3+hold

EEPROM dump: power+1+9+hold

Firmware version: power+2+5+9
Key touch test: power+0+6+hold

Frequency step test: power+1+5+9

RS232 PC test: power+0+1+hold

UBC-3500XLT shows opening message text a few seconds before entering test mode.
If after power off and then power on 3500XLT still shows test mode press scan key to exit test mode.

LCD pixel test

 

Frequency bands test

 

Firmware update mode

 

EEPROM dump

 

Key touch test

 

Firmware version

 

Frequency step test, it overwrites custom search ranges