Don't make the assumption that you have a drain. If it does it again, take the battery out and to the store where you bought it and have them check it. Just because it is a new one off the shelf doesn't mean it is good. Before you take another out of the store, ask them put it on a load tester to see if it is OK. I had this problem last year with a brand name battery bought at Schucks - a national box store parts supplier. I went through 3 batteries - each time they said it had a bad cell and they replaced it. Last time I went in, I asked them to load test the new one - it failed right off of the shelf. They made it right by giving me a new Exide gel battery and I have not had any problems. Before I figured things out I pulled my hair out trying to figure the nature of the drain. I had one guy tell me that if you are going to purchase a corrosive battery, go to an Interstate dealer as they usually turn the stock quickly. The box stores buy them in bulk and sometimes they sit on the shelf longer than they should.
Good luck.