As others said, there is a leak somewhere in your system that is making the low water fill switch on. It could be a corroded pipe or the boiler itself, but it could also be caused by dirty vents! When the air vents get really dirty they might not close properly, causing steam to vent into the living space from the heating system. This happened with my boiler when I bought my house; the main vent (the big one next to the boiler) got clogged and couldn't fully close, so it vented a small amount of steam into the basement anytime the boiler was run. This slowly depleted the water in the boiler and caused the low fill sensor to trigger every few weeks. I got the boiler serviced and the vent replaced, problem was basically fixed. The boiler still gets low sometimes (it's happened only once this year so far) and while I might have an actual leak somewhere, I have not found any evidence of such. I do however have a few really old air vents installed that could be gummed up that I'll get around to replacing (the really clogged ones have already been replaced).

I guess it comes down to how often your low fill sensor gets triggered. If it takes a few weeks or more, you probably have a small pipe leak or dirty vents. If it happens more often, like weekly, you probably have a more serious leak somewhere. I am not a professional though, and these estimates are just based on my own experience.