If someone overfilled it, it could be burping out the dipstick tube and running down the back of the block. My Crown Victoria burps out the dipstick tube occasionally for an unknown reason. If you're sure it's coming from between the engine and transmission then geezer is (as he always is) spot on, there's really not anything to do but take it apart and find the culprit. I wouldn't pay a ton for it, always the possibility you'll have to buy a transmission.