The first question is whether the gauge has moved
Before identifying the water, identify whether the system has lost any. Note the pressure now, then again after the boiler has run. Ideal put the correct cold figure between 1 and 1.5 bar.
That single reading splits the problem in half. Condensate is not part of the sealed heating circuit, so a condensate leak drips steadily and leaves the gauge exactly where it was. Anything carrying system water, whether it escapes through a seal, a cracked heat exchanger or a lifting relief valve, takes pressure with it.
The four sources, and how each one looks
Condensate: clear, constant while the boiler fires, no pressure change
A condensing boiler produces slightly acidic water as a by-product, which Ideal describe draining away through a white or grey plastic pipe that runs part of its length outside. If that pipe blocks, cracks, or freezes, which Ideal note happens around 0°C and below, the water backs up and finds its way out of the bottom of the case instead. The signs are a thin, clear drip that appears whenever the boiler runs, often with a lockout, and a pressure gauge that has not moved at all.
This is the one homeowner-fixable case on the list. Ideal's own guidance for a frozen pipe is to remove any insulation and warm the pipework with a hot water bottle or a microwavable heat pad, then press the restart button. Never use boiling water on a frozen pipe, and if it refreezes every cold snap the pipe needs insulating or re-routing.
Relief valve discharge: often no visible drip inside at all
The pressure relief valve is plumbed to a pipe that, as Vaillant describe it, runs from the boiler to the outside wall or down towards the ground. Ideal say that pipe should normally be dry. When the valve lifts or passes, most of the water goes outside, but a badly run or partly blocked discharge can back up and show under the case. The tell is that the pressure falls, and there is a matching wetness outside.
What is behind it is usually not the valve: it is a failed expansion vessel over-pressurising the system, or debris in the valve, which Baxi name specifically and say needs cleaning out or replacement by a Gas Safe registered engineer.
Seals, joints and the pump: system water, slow, pressure falling
Vaillant list leaks from seals among the standard causes, noting that they loosen or break over time. Inside the case that means pump head gaskets, the diverter valve or hydroblock body, the automatic air vent at the top, and the many compression and O-ring joints on a densely packed combi. These produce a slow drip that pools under the boiler and a gauge that creeps down over days. Corrosion staining and limescale crust on the pipework under the case is the visual signature.
The heat exchanger: system water, worsening, often the end of the boiler
Vaillant list a failed or cracked heat exchanger, and note it is more common in older boilers. This is the expensive answer. Depending on the model and age it is a significant part cost against a replacement boiler, and it is worth getting a written diagnosis rather than a verbal one before deciding.