Boiler Resevoir Slow Leak
Eventually if water continues to fill the system, it will fill the pipes, and eventually come out the vents on the radiators.  I saw a house that got flooded when someone was adding water to the boiler manually, forgot to turn it off, and left - made a big mess.  Obviously that would take a long time with a slow leak.  

I am not sure whether this also creates a safety issue.  

There may be an automatic fill valve that isn't sealing off properly when the water reaches the full level that can be serviced or replaced.  I believe these can also be retrofit if there isn't one on your furnace.