I would have thought the whole point of a condensing boiler is that the
exhaust contains little or no water vapour.

This would only happen if the flue is long enough for all the water to be
removed, or the return temperature is very low. If the return temperature is
around 60C, then the boiler fundamentally can't cool the flue gases any
lower that this. Those gases would then still contain water that will only
condense out when the temperature drops further, either within the colder
flue (if it is long enough) or upon exposure to colder air (at the
terminal).

Any condensing on the flue or outside air is wasted energy. That is one
reason why greater efficiency occurs at lower return temperatures. More
condensing can then occur upon the heat exchanger itself, where the energy
can be usefully recovered. (There are also thermodynamic reasons for greater
efficiency at low temperatures that apply to all boilers, not just
condensing types).

Christian.