I would check constantly because they get a little careless as time goes on. there are ways to get around drug tests, mine did it. I know for instance heroin only stays in the system around 3 days, so if he's using something like that, but not daily, yet, then maybe his testing just happened to be at the right time when things weren't in his system.
Just keep your eyes and ears open and watch for different signs. It could be that he's onto the fact that you may be suspiscious and he's being more careful, or maybe you are wrong and the pens are for something else. Either way, don't let your guard down too soon.
I pray for you and him that it is nothing, because I wouldn't wish the nightmare of dealing with drug addiction on my worst enemy.