It could have been a compressed code—maybe if Minerva had sent the message to Dumbledore there would have been no prefix.
Also, as the Time-Turner goes back in hourly increments, some additional security could have been achieved by making Harry use it in front of Minerva, and monitoring the room in question at 8PM, 7PM etc. It’s possible that might cause or be purposely made to cause paradox, though.
It could have been a compressed code—maybe if Minerva had sent the message to Dumbledore there would have been no prefix.
Also, as the Time-Turner goes back in hourly increments, some additional security could have been achieved by making Harry use it in front of Minerva, and monitoring the room in question at 8PM, 7PM etc. It’s possible that might cause or be purposely made to cause paradox, though.