In my experience, the Montgomery County encrypted channels are not even detected by my GRE, RS and Whistler scanners. For example, I can manually set the scanner on FD TG 7L and I don't hear anything when there is a consultation underway. When it scans, it doesn't stop on 7L even though there is (apparently) a consult there. In contrast, there are other digital trunked radio systems where you can hear encrypted gibberish that sounds like analog scrambling, particularly on Federal Government digital trunked systems. So there must be different encryption technologies being used.
Not to a great extent. Depending on system attributes, encryption is typically DES or AES, both being applied over the standard P25 waveform in the same way unencrypted voice data is transmitted. [In technical terms, the voice data is encapsulated into P25 packets the same way regardless of whether it has been encrypted or not prior to encapsulation]
The big difference is that different system types will have slightly different sounding audio artifacts when a non-filled portable or scanner decodes the encrypted P25 voice as non-encrypted. TDMA (Phase II) encryption noise sounds noticeably different from Phase I FDMA when decoded from the P25 stream, and I've heard slight signal sound differences between encrypted voice on a P25 FDMA system and an old SmartZone system using P25 voice.
If your scanner is failing to stop at all on that talkgroup, that's likely a function of the scanner. Regardless of system type, if I set any talkgroup on my scanner as a regular P25 talkgroup and let it try to decode, it'll stop-scan and let all the garble through. Some scanners (GRE has this option, not sure about others) allow you to pick whether you want encrypted traffic to be silent, busy-toned, or passed as noise on a radio-wide basis. My portable does the silent-treatment by itself if it loses its keyfill, and in those cases typically looks like the talkgroup is not in use (like you described).
P25 encryption does not change the characteristics of the voice transmission itself - the voice is still encapsulated and transmitted as regular P25 voice (having already been encrypted by the portable), with the sole addition of encryption type and algorithm ID info to the P25 voice-integrated-data stream.
Now, if you want to go back in time to talk about DVP or Securenet, then you start to get into very different ways of doing it.. but then you're not using P25 at all anymore, and that's not why we're in this thread
