ENCODING (which is what P25 is) is simply the conversion of voice into digital packets before they are sent over the air. There is no secretive-ness to it. Sure, initially, when P25 became popular, and there were no digital scanners on the market, it made monitoring these systems quite a chore. But it was NEVER meant to secure radio traffic. It's simply an alternative way to send voice over the radio waves.
ENCRYPTION is a process of turning voice into digital packets, then "mixing them up" before sending them over the air. All radios must have the "key" to put the packets back into the right order, so they can be converted BACK into audio. There are hundreds of millions of combinations that can make up the "key" and most modern encryption systems can re-key over the air at any time, making it impossible to "break" the key and intercept the audio.
But what is to stop a programmer writting a software to scan key combinations .Computers can do millions of combinations in minutes, people doing it take long time.
No scanner can monitor an encrypted signal. Further, it is unlawful to monitor encrypted signal - in many cases, it's a federal crime.