As other poster says make sure you have a good connection at the battery, cable ends look OK? Sometimes there is corrosion under the insulation.

Yes you could have a drain, quite a big one if it discharged the battery overnight. Assume you have checked all lights such as interior, trunk, underhood, glove box to make sure they are out. Next step is to detect the drain with an ammeter connected in series with disconnected battery cable and battery. Should not be more that 150 milliamps or so. You could also use a small 12 volt low wattage bulb such as the ones used for dash lights for same purpose. If it lights you have a drain in process. Next step is to pull each fuse 1 by 1 to see which fuse stops the drain. You should then be getting close.
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David Hunter