[lmilcin]

The grinder is Baratza Sette 270W. The espresso machine is Rancilio Silvia.

The espresso machine has no low voltage electronics, it is basically just switches, thermostats, a heater, a pump and and a valve. It has two inductive components, a pump and a solenoid valve with no protection. This is a recipe for huge amount of EMI when the devices are turned off (when inductive component is turned off at a right time the voltage rises and can cause spark and high voltage that is applied to the outlet).

Since the grinder is connected to an outlet close by it is subject to EMI. I have not looked inside the grinder but there there are components like thyristors and triacs which are used to keep a device turned on until it decides otherwise, that can get turned on by EMI. Generating enough voltage can cause a thyristor or triac to turn on by itself which is then enough to keep it turned on and cause the device to wake itself up without the user pressing the button.

I will also add that I have two versions of the grinder. Only the older one is affected by the problem. I find it likely somebody spotted and fixed the problem (it is extremely easy to fix just as it is extremely easy to overlook).