If it was urine, you would definitely be able to smell it immediately. The best way to clean it up would be to go to the store, wal-mart always has everything cheaper, and purchase some interior upholstery cleaner. Use a damp white washcloth and get some water on the seat before you use the cleaner, that way you can get some more of the "mystery liquid" out of the seat first. Then follow the instructions on the upholstery cleaner. Let your car air out for a day to two and then you should have no more problems. If its a urine stain, look for a cleaner that says it specifically will take out urine odors.
