Many, but certainly not all detergents contain optical brightners to make clothes appear cleaner, and in general those optical brightners do fluoresce as blue under UV light. In order to useful however, they have to be able to survive rinsing, so in some respects they are much more like dyes than detergents. Unfortunately all their presence indicates is that at some point either the garment, or the thread or material that went into it were washed in a Detergent with optical brightners. In other words it has been washed, but there is no way of telling whether when in it was washed. It could be anytime from the time the threads that are used to make the material were produced, to after initial sale, and before return. So unfortunately the presence of optical brightners that fluoresce tells you very little other than the fact that it has been washed.
