I am a plumber in the Utah area and this is what it sounds like to me when they installed the grout they didn't do a very good job he came back and fixed that now what do we know about grout? Well grout is sand and what happens to sand over time that its exposed to water? Well it starts to break away and breaks down now the water seeps thru and fills up your shower pan and overflows causing leaks! it lasted for 2 years? that's about how long it would take to break it down you are suppose to seal the grout every 5 years I recommend every 2 it probably never was sealed I would plug the drain with a test ball and fill the shower up with hot water and let it sit for a while and see if it leaks if it does wallah if it doesn't you know that your leak is somewhere in the plumbing drainage to avoid it could be coming from shower head or water etc fill it with a bucket from another tap. hope this helps.