When car is in transport mode, why is the dash batt symbol flashing red?

It should last 3 months. But you can buy a battery tender, very inexpensive and hook it up. It's a very small trickle charge that will not overcharge your battery but keep it tip-top. But you have to have it parked where a regular 110V household power supply can be reached. Otherwise, 99% it will be fine. Fill it up to prevent condensation and even some Sta-Bil wouldn't hurt, although not mandatory

But red flashing battery symbol icon? The one that actually resembles a battery? Or just the small red LED security light? The clock and security draw very little power, a good battery 5-6 months