You can put a leak sealant into the system but this will entail draining down the heating system. This may only work for a short space of time as well.
Having to blead a radiator more than twice a year is not good for the heating syatem, as you are introducing oxygenated water into a system with steel radiators/convectors. This will lead to the possibility of sludge, rust build up and invetable leak.
There may be tiny leakage from gland packings in valves especially when the heating is off during summer months. There may be a system water discharge from an open vent or pressure relief valve. The coil/heat exchanger in your hot water cylinder may have perforated leading to heating system water loss.
Either way I would recommend that you get it seen to as having to bleed a radiator regularly is not good for your system of your pocket in the long run.