Do a resistance check between your positive terminal and the chasis with the car switched off. Sounds like you may have a partial earth.
Set your multimeter to the "Ohm" setting. Hold one terminal on positive and the other on a nice clean bare section of metal (Engine block would be good)
Reading should be over 100M ohm. If its significantly less than this you have a partial earth and the battery is slowly discharging to earth through the chassis. Also check all your headlights, tail lights and indicators, if they are flooded or have water in them that can also cause earths.

It may also be the battery. If its quite old you may need to get a specific gravity check. The electrolite might be shite.... Normally when lead acid batteries get old you have to do whats called a 'gassing charge' on them which involves overcharging the battery. Its easier just to get a new battery.

If the new battery goes flat in the same manner, its defiantly the problem that i stated earlier.

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STi Ver 6 | 2.1L Stroker | 418whp | 11.5 @ 199kph |

Not fast enough dammit!