Hi Napalm and all,
"Putting 9v into something that's rated for 12v *probably* isn't going to damage the scanner."
It won't damage the scanner, no "probably" about it. Trouble is the performance will be degraded, particularly programming and memory which require a stable power source and 9V is at the very lower limit. There are internal voltage regulators to consider and the input must always be higher than thier output for them to function.
"Putting 15 volts into something that's designed for 9, probably will."
With a 9V device I would expect smoke signals saying goodbye. With 12V it's no again, but that's the upper limit and you really don't want to push your luck. A scanner is intended for mobile operation and such equipment is designed to operate in the 10-15 volt range with 13.8 being nominal. This is because an automotive electrical system operates this way. 12.6V being a fully charged battery's terminal voltage with the engine off and the regulator typically charges it at around 14 volts.
The bottom line is always use the proper supply for the job or you're asking for trouble one way or the other. You can always go for overkill in the current department, the more amps the merrier but watch the voltage. You don't want to end up like that traveling gnome that plugged the American appliance into the European outlet.
Now somebody mentioned magic smoke. It's only good when it's of the herbal variety, (the police may have other ideas) but when it's from electrical components it's bad, very bad. Smoke is put in at the factory because it's essential for operation but unfortunately you can't get it back in and no replacement smoke is available. When the magic smoke escapes it's all over Jack, game over.