Salt does not evaporate. The only way you would lose salt is to drain the pool, a leak, or when you splash or bring it out on your body. Whenever your system says the salt is low or high, you need to first curb the urge to correct it without a test first. Your sensor, when it gets the build up on it, will skew what the unit interprets as an accurate salt reading. That's not to say that cleaning will correct this every time. As stated above, those sensors are very problematic to say the least. You state that your sensor is 6+ years? That is great, it is also the exception, not the rule. More than likely, you are up for another soon, if not right now.
If your system had been working then all of a sudden, boom, your low on salt, be skeptical. Two bags may, or may not, have put your salt level too high for proper function. Yes it may be still functioning but the system tells the cell to come on based on IT'S results, and running the cell at higher salt levels will damage the cell. Now if your cell is also 6 years, you are probably ready for a new one of those too and that could be the reason for the trouble you are having. One thing i have learned from these units is that in a lot of cases, the code it gives you could not be farther from the real cause.
I recommend to customers that do not have a pool store locally, or one that they trust, to get their own salt tester. I DO NOT recommend the strip testers. Digital salt testers can be had for +/- $100.00 on line.
Here are a few...
Amazon.com: pool salt meter
