security systems in a 1987 300zx

OMG I just got into my first sports car ever WOW!!!! I love it... Just a question to anyone that will help. I have no idea about the security system in the car. I opened the door and sat down to clean the car out and the whole car shut down.The key was in and turnned but not running..checking lights etc then nothing would work it was dead. I disconnected the battery and got it running again. Then for no reason it did it again, I got out of the car for about 5 min, went back opened the door and yep you guesed it FLASHING LIGHTS, HORN GOING OFF boy oh boy my thats not fun..... someone help me before someone thinks IM breaking into the car, and I go to jail.. thanks Jenn