If this is a vanity I would check the plumbing for any leaks. Could also be dampness, are these particleboard cabinets?

Yes it is a bathroom vanity. The inside cabinets and pipes do not show any signs of leaking or dripping. The cabinets are KraftMaid and presumably particle board with veneer.

You could try putting some tissue paper along the toe kick. Then use the sink. If it gets wet, you have a leak.

could the floor have been wet when they installed the cabinet? Looks like the stain could have bled onto the tile and the water soaked into the vanity (if you don't find evidence of a leak now).