How could there be nitrates present in this tank??? I'm confused. :B?

Sooo, last night I had a customer come in with a water sample. She had purchased her 75 gallon tank off of craigslist. It had been sitting dry for quite a while before she bought it. She washed the old gravel out in a diluted bleach solution. The filter was brand new. She ran the tank for about 3 weeks with nothing in it--no chemicals, no fish, no plants, nothing. So on Saturday night she came in to get some fish. I didn't sell them, obviously, or I would have pushed her to do a fishless cycle, and I would never have sold her as many fish as she had gotten if she had insisted on doing a fish-in cycle. She got 3 fancy goldfish, a shubunkin, and a pleco. Needless to say, all died over night. Last night I was working when she came in to return them. I did a test on a water sample she brought in. Here's the part I don't get: nitrites were 0, ammonia was barely detectable, maybe .25, and nitrates were 50??? I did a retest, thinking that couldn't be right. Same results. How...HOW??? I don't understand how any of the bacterial colony could have survived, yet these are the results I would expect from a newly cycled tank. Any thoughts...?