My salwater fish turned absolutely fresh water! Is this strange or what???

I know this is really weird, and I am the type of person to usually be on top of everything when it comes to my fish, but I've been working and schooling full time each so I am never home much. Anyhow, I have a lid on my tank that is raised about 5 inches from the top of the tank, and it therefor evaporates water FAST. Once a week, I or someone lese has to fill almost two inches back up. I haven't tested the salt water level since I moved in almost a year ago, and in thinking that salt DOES not evaporate with the water and the fact that my fish are thriving, I procrastenated. I finally tested it, and it was completely SALT FREE. I never thought it could be pssible, but it is, I bought a second tester to double check. Has this ever happened to anyone before?