What would cause my battery to drain intermittently for no obvious reason?

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Car was starting up and running great. I stopped for gas and then it would not start. My first thought was the starter went. It did not turn over.. just click, click, click. Towed to my mechanic who jump-started it.. checked out voltages (battery, alternator, starter).. all good. Drove car 40 miles to work no problems. Started car 1 hour before shift ended, again it started up. One hour later when it was time to go home.. DEAD. Had to get a jump and drove home no problem. I let it sit for 3 days assuming that there was a drain and it would not start.. but it did! Asked my mechanic to check everything.. he did and said he could not find anything. I KNOW there is something wrong. When at a stop light my blinkers slow down as well as my fan. Am I wrong to think that I need a new alternator? What could make a battery suddenly die within an hour and then come back to life?? Im saving for a newer car, but I do need to rely on this parasite to get me to work. I welcome any and all suggestions. Thanks !!!