"After being on battery all night..." If your system lost AC Power for an entire night, and IF you tried to use it as soon as AC Power was restored, then the results are unpredictable. Your battery was probably very weak in the morning, and was drawing heavily on the Panel's (The CS-270's) on-board power circuitry, which can draw down the voltage/current available for the Panel to perform other operations---like, for instance, its ability to process Keypad input and Arm or Disarm the system. Some Panels won't disarm if the battery is too low, and that can lead to a lot of different and confusing symptoms for the same problem: Low battery. If you haven't changed the battery in 5 years, get a new one now. I recommend the 7 or 8 Ah battery.
If you still have a problem after waiting a day for the battery to recharge, IF it's a fairly fresh battery (no more than 3 years old, should read abt 13.2-13.5 Volts, disconnected from Panel), then post back and describe what symptoms you're seeing now.
BTW, when you say, "...PIR lights are blinking", I'm guessing you mean the Keypad's zone number lights for those zones that have PIRs. If you meant the lights on the PIRs themselves, please make that clear: Different symptoms entirely.
The Armed light blinking indicates an alarm in memory, and "Activating the PIR makes the 3 lights blink again." probably means that the system was still Armed and each PIR activation initiates a new alarm. As I said before, your normal Disarm code might not work until the battery is charged.
Please post back and let us know what symptoms, if any, you still have.