Battery keeps on draining out overnight

The battery will not stop draining out overnight. I replaced the battery it was not the issue and I have had the alternator and battery checked by different sources to make sure they were both running well. So it is draining out and it is not from battery or alternator. When I jump it everything works fine only thing is I notice that when I jump it and it turns over as soon as it starts turning over the horn goes off once. Also, in the past the car alarm has gone off randomly at night. This is a recurring problem over the past six months. It went away for about three months and now it is back. Other things, when it shuts off sometimes it fully drains the battery other times everything will turn on in the inside, but it will not have enough juice to turn over. I am not great with cars so I do not really know what to do now, hopefully someone has some sound advice. Thank you

other things to note; I just jumped my car and couple things happened, one as soon as the jumpers touched my cars battery the horn also sounded off. Also, I did not even have to start the other car I was just able to start mine with that little bit of juice. So I am thinking electrical? Maybe it thinks a car door is open but I have open and slammed them all shut on other cases also the alarm should not go off considering the car is unlocked. I am going to run it for a while and try to watch it closely when I do finally turn it off. I will probably be going to Autozone maybe see if they have anything I can hook up to my car to see if its an electrical problem somewhere.