Boiler unexpected startups

I have a Vaillent EcoFIT 630 system boiler with Megoflo Unvented cylinder. The system has Nest thermostat installed recently. The issue is at a random time of the day, approximately once a day, boiler comes on and go into heat and idle cycle (when max water temp reaches) without Nest thermostat showing anything that boiler is running. On nest, there are no schedule jobs. The heating mode is off. I don't think that boiler has any eco mode (as it is system boiler) When the boiler unexpectedly turns on, it goes to the heat and idle cycle for hours. Sometimes it stops itself, some time I stop it manually. While it is running, Nest app or thermostat doesn't show anything running and I cannot control anything from thermostat during that time. Any suggestions or leads are gratefully welcome.