Residual voltage at the output of smart plugs (Teckin SP21, Tapo P10, GreenPower PC-03)

Hi,
I wanted to use a smart plug or timer at home, I have 4 different models. After plugging each of them into an outlet at home and switching them off, I expected to have 0 volts at the output, meanwhile at the different outlets at home I have several to several volts at the output. Maybe my house is suspicious, but I repeated the same tests in the company at several locations (regular measurements of the correctness of the electrical installation are made) and I also have a supply of 20 volts - 50 volts at the output. I don't want to believe that the installation is NOK or the plugs are faulty. At a friend's house, one of them gives 0 V at the output when switched off. What could be wrong?
Appliance models:
GreenPower PC-03 timer
Smart Socket Teckin SP21
TP-Link Tapo P10

And what are you measuring this voltage with?
Greetings. J.

>>21760715
Multimeter ~230V.

Hello,
i don't know the design details of these 'smart' sockets, but I'm afraid you simply have the relay or triac contacts bypassed with a capacitor and it is this that causes the small current flow that you observe.
A solution often found in timer switches with clocks, e.g. mechanical timers.
For example, have you not observed that a connected LED garland, for example, still lights up slightly despite being switched off?
Greetings

>>21760735
I would like to control the circulator, but there is some electronics in this pump and when it sees a voltage > of about 5-10 volts it flashes a diode, according to the manufacturer this can damage the electronics and I prefer not to take the risk.
What is puzzling is that every object / socket I have a different voltage at the output when the socket is switched off.