You can't see all the pipes.
If you can't see them, suspect them. This is especially true of buried lines. Condensate contains a mild solution of carbonic acid that can eat through steel. If you're not venting the system properly, the level of carbonic acid will be higher than normal. If a pipe is buried in concrete, the concrete can be aggressive enough to put holes in the pipe.
If you can't see the pipes, suspect them. If they're very old, replace them.
The main vents or radiator vents are losing water or steam.
You can lose a lot of water through an air vent. In apartment buildings, tenants sometimes remove the air vents from a radiator or two. They like what the hole left by the missing vent does for the humidity level in their apartments during the winter. Remember, though, what you lose through the vents you have to make up at the boiler.
If the system isn't well vented, the velocity of the escaping air will increase through whatever vents are working. High velocity air carries dirt toward the good vents and clogs them. If the vent can't close tightly because there's dirt in its seat, lots of steam will escape. You'll have make up that missing water at the boiler.
Make sure all your vents are working properly.
The system suffers from water hammer.
Water hammer is so destructive it can actually pull pipes out of fittings. It can also destroy air vents and steam traps. Water hammer blasts open holes through which water will flow to the outside. And then you have to make up that water back at the boiler.
Get to the source of the water hammer and get rid of it.
There's a hole in the boiler.
Now this isn't a hole you can see. It's an internal hole cause by oxygen corrosion. It happens to cast-iron boilers when they take on a lot of fresh feed water. The oxygen boils out of the water and eats a hole through the metal right at the boiler's water line. The water steams off and goes up the flue.
You can't see it unless you're looking at the chimney. It looks like white smoke, but it's not smoke; it's water vapor. To check for a hole, flood the boiler up into the header piping. You'll know you have a problem if you see water pouring out of the boiler's jacket.
Check under the jacket for cracks in the boiler that may not be large enough to cause a flood on the floor. The water may be evaporating on the hot cast iron. Is it unusually humid in the boiler room? If it is, there may be a crack in the boiler. Check it out.
And by the way, if the boiler is leaking, don't' try to fix it with dope. That stuff may stop the leak, but it will also contaminate the water to a point where the boiler will produce nothing but wet steam. You can't heat a building with wet steam.
The near-boiler piping is all copper.
Copper headers come apart when the expansion caused by the hot steam starts twisting them. This is especially true if the boiler has two or more risers to the header. As the soldered fittings let go, the steam begins to leak - and you can lose a lot of water through a leaky soldered joint.
Be smart and pipe your steam boilers with threaded steel instead of copper.