pre-finished wood floor

Ok, what's under the flooring?
Slab, crawl space, basement.
Any vaper barrier under the flooring.
If there's a crawl space is there a 6 mil. plastic vaper barrier on the ground?
What type wood is it.
Is it changing color near a window that gets direct sun?
Was a moisture test done before installing the floor?

Very likely caused by the sun---You did not mention the type of wood----Maple and cherry both darken within a year or two----

I agree with Mike. THis was probably caused by exposure to light. Some woods will darken over time and with exposure to light, while others will lighten. If you are able to post a picture, we could probably give you a good idea on whether or not this is the cause.

i put brazillian hardwood down in my house and within 3-4 months it had darkened to a nice reddish brown from the sun and the natural oils in the wood, but we knew that before hand that is why we chose it. i vote for sun exposure also.