Can you set yout D3 to prompt for a password

on startup to prevent unauthorized used (valet or parking attendants)

Nope. But IIRC you can set it up with a password so that if it gets stolen, the next time power is hooked back up to it, it won't work without the password. But that's only if the constant power to the unit is lost. And I don't think losing all your presets and settings is worth being able to do what you're asking about.

I guess you could wire in a hidden switch on the accessory wire, so the unit won't power on unless you flip on the switch. That would save you the hassle of having to put in the password every time you get in the car. Just turn off the switch before you hand over the keys and whoever is driving it can't use the radio.

Thanks for the reply and suggestion.