The question is how old is the battery?

Your alternator is good as the mechanic said. Perhaps your battery can no longer hold when the battery becomes discharged, it will cause a small about of water to evaporate. Remember that the battery is filled with diluted acid. Over time, the water level drops to such a level that it won't hold a charge.

Maybe this not your case, could it been your battery cable was loose, dirty during that time when you were driving and all the juice that you need are run out, causing your car chocking and wont start anymore.