I suggest that you contact Frank Deangelo and have an inspection done, he is very through. He normally has an ad on the site, but if you search, you will find his contact info. We and several neighbors have used him and were very pleased.
IMHO, I am guessing there is a small crack in the slab, and it is showing up in the tile. Your biggest problem is probably going to be how to match the existing tile. Home warranty should have the original tile, and with some luck you may be able to find some.
Hope this helps.
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Pennsylvania, for 60+ years, most recently, Allentown, now TV. 
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