Siren and Keypad sounds with 15 MINUTE delay

My Extra Siren and Keypad Remote keeps beeping at the same time 10-15 minutes AFTER I have set the alarm system in home mode. Once the countdown has stopped and I get the announcement from the base station then 10-15 minutes goes by and the Siren and the Keypad Remote starts to beep at the same time. I’ve replaced both the Siren and the Keypad Remote with new batteries. This system is not the original one, I’ve had this new system for only one year.  I’m at a loss why this keeps happening. Any ideas? I need help with this.