The boiler produces a realy big plume of water vapour but I have never
noticed any water coming out of the condensate drain pipe.

I don't know about your specific model. However, many condensing boilers
have a syphonic trap for the condensate. Rather than coming out as drips,
about a litre or so will be released in one big hit. This helps prevent
freezing of the pipes, as the quantity tepid water won't freeze on contact
like the drips will. You might have a boiler with such a trap and might not
have been watching when this rather infrequent event happens.

Christian.