yellow spots under linoleum

The vinyl floor in my mother-in-law's bathroom was put down in 2003. In the last two years, small bright yellow spots have appeared on it randomly and seem to be coming from underneath the vinyl. It's driving her crazy, so of course, it's driving us crazy. Any idea what would cause this? Is it possibly some reactiion to nail or screw heads in the wood underneath?

Water or moisture, under the linoleum could possibly, cause that problem, especially, if you notice it increasing in frequency as you get closer to the toilet. Mine was blue spots.