His lawyer claimed it gets stored in fatty tissue and released when that tissue eventually gets used by the body. Since the newest tests revealed "trace amounts" I can imagine a scenario where a trace slightly over the level of detectability might show up in a test whereas the level was below the threshhold previously or within a margin of error. But I think it unlikely that only now does his body release more of any stored chemical.
The possibility that he is so stupid that he used it again seems almost impossible, but his uncle may have been even stupider. Perhaps he and his uncle did it the first time together, and decided to try smaller doses that eventually built up. But I doubt if any dose that was at all effective would avoid detectability.
I wonder one thing that might explain it if he is innocent--the possibility that the testing has become either more sensitive or more precise, detecting that residue in his system which evaded detection previously. Changes in handling, different testing materials (which could have subtle differences unknown to cause changed results), and even methods of analysis could all play a role. Labs are not infallible.
I had a scare 6 years ago when high contrast CAT chest images taken for my heart revealed small nodules (2-4 mm) in my lungs. Never smoked, but the protocol was for cancer screening follow-ups after 3, 6, 12, 18, and 24 months. Then we found that the protocols changed because all types of non-smokers were being found to have these nodules but they were seldom cancerous (<2% I think)--the high-contrast CAT was able to resolve down to that level, and previous test equipment had only showed 4 to 6 mm. The researchers concluded that rather than detecting cancer earlier, as the medical community had excitedly believed, the followups were endangering the patients more than the nodules, and followups were cut in half (no growth, incidentally). It was a complete reversal of the medical opinion.
I doubt the tests have changed, but I would give the kid a polygraph if he wanted one. Generally unreliable enough so they cannot be allowed in court, but in this situation, if he insists on not having used, I'd give him a chance.
Incidentally, I happened to be volunteering in the medical library at the local VA hospital when that lung issue surfaced. I researched the heck out of it, reading all types of professional periodicals, and found that Journal article that first recommended the change in protocols, and gave it to my cardiologist. It pays to educate yourself when facing anything medical--my doctor has hundreds of patients to educate himself about; I have only one.