Possible water leak under hardwood floor

I think your implied question is: How can I determine if this is an active problem or is a one-off from a previous incident?
Solving water issues often requires detective work. It's hard to tell from the pictures but from your description it sounds like minor water damage. The first step is to check for a logical source of the water. Remember, it's possible it occurred during or shortly after construction. The pictures show the damage to be mostly away from the wall. You might check for signs of a leak around the radiator but it's unlikely to be the issue unless there is floor damage close to it. The other possibility is a roof or skylight leak directly above that spot or even a potted plant that was over-watered at some point. With a skylight leak you may or may not see signs of moisture around the skylight - watch for drips during the next rain or snow event.
The important thing is to check for moisture currently present in the flooring. Often it's hard to tell without a two pin moisture meter. They're available at most home supply stores and aren't too expensive. That will give you an idea of how recently the problem occurred. If a meter shows no sign of unusual moisture content it will at least allow you some piece of mind. If there is moisture - keep detecting.