Only come across similar thing once before
Rug had been left on stone flooring whilst the housekeeper had mopped floor........hence the edges and corners had weak colours due to ammonia/bleach in floor cleaning product :
You cannot check whole rug for test but check PH on the bad areas and check the placement of the rug for what may have been used to clean around it. Laminte may mean soapy edges, Stone may mean bleached edges and white carpet may mean
that it has not been moved when carpet was cleaned at some time