From what I remember that is a combi boiler with an Eco button, with Eco on the boiler should only work when there is a demand for domestic hot water or central heating, with Eco off it heats a small tank inside the boiler so the water is instant, this means the boiler will fire up every so often.
It seems the boiler changed over time, later models can have a special Bosch thermostat with modulates the boiler with connection to ebus, early models can only use return hot water to control output.
The theory was the boiler pumps hot water around the system, if all TRV's are open then the water is cooled so the return water is cold or cool so boiler runs flat out, as the TRV's close the return water has a smaller and smaller route and finally it will open by-pass valve so the return water gets hotter and hotter which causes the boiler to modulate (turn down flame height) and then cycle off/on.
So in theory it does not need a thermostat fitting. However in practice it will continue cycling on/off all summer, so normally a thermostat is fitted so as summer arrives it will turn off.
So in your case it would seem it's the heating of the domestic hot water causing it to fire up, try pressing Eco button that should stop it. I found Eco set on the taps can be cracked open to give a trickle of hot water, Eco off and taps must be wide open for boiler to fire up, Eco on resulted is shower getting warm then cold then warm again as boiler tank ran out, eco off once warm stayed warm.