What he means is when you draw cold water from anywhere, if the mixer shower is faulty(ie passing from hot to cold) it will draw water from the hot supply which will fire the boiler.
No it won't, as the pressure on the hot will always be less than the cold that is the supply. Unless the hot is boosted in some other way independent of the cold supply that feeds the combi, and that would be very rare. I've not seen anything like that in 30yrs.
As said earlier in the thread, the issue the OP is experiencing is most likely caused by an air pocket trapped in the pipework, this could be from an old capped pipe, or even something like a tap that's not been used in a long time in an unused bathroom for example.
Also as mentioned earlier, a typical fix if the air lock cannot be removed, is to fit a non return valve on the cold inlet to the boiler, this will stop the water pressure relaxing and then compressing (the air pocket) which causes the boiler flow switch to operate.
You also need to be aware that in most cases doing this will invalidate the boiler warranty as the volume for natural expansion is severely reduced, and can exert excessive pressure on the boiler hydraulics through normal heating (of the DHW) operation.
In certain circumstances, and Baxi is one, they will allow you to fit an expansion vessel with the non return valve, to absorb the expansion, and keep the warranty status on the boiler.
