Chuck, the voltage you read is the meter creating a continuity between the post and the disconnected neg battery cable, which really doesn't tell you anything. As another post mentions, you need to determine the current consumed by your vehicle that is different than normal (note that the car consumes some nominal amps when off). You will need an ampmeter, and one that is rated for enough current to keep from frying the meter if the short is a large enough one that exceeds the capicity of the meter. Since your not blowing any fuses, a 10 amp meter should do, but you can also try pulling out the fuses that operate systems that are on even when the key is off, like interior lights, emergency flashers,parking lights, etc and wait to see if the drain stops. Also remove the 25 amp fuel injection fuse and/or the accessory positive wire from the battery in the engine compartment to see it that isolates the short.
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'89 245 Sportwagon, '04 V70 2.5T Sportwagon