Well you are the second user to report this. So far.
Normally arming an X10 Security Console should not sound a Powerhorn, as it triggers on a cycle of On Off or All Lights On All Units Off commands. The PS561 in the older DS7000 Kit. Sends the All Lights On all Units Off Command so you could set the Powerhorn to an unused unit code and everybody was happy.
The new SC1200A sends the Light Code Addresses then an On. Light Code Addresses Off. So you have to keep the Powerhorns on one of the two Light Code Addresses. For them to work.
So is it normal. My opinion No. Arming your system should not wake up anybody sleeping. Is it how the SC1200A and PH508 {depending on the age of the pH508} going to work. Very possible.
My tests with different date code PH508s show different results.
Can you provide the date Code on yours? Should be a small white sticker on the back cover. With two numbers then a letter and two more numbers.