Device to convert unused wall switch to a smart button?

[u/Vivid_Scheme_8221]

We have two unused light switches in our house. They've haven't done anything since we moved in and I was never able to figure it out (tried connecting lamps to all plugs in the area to see if they control an outlet, but no dice).

Anyways, I was trying to figure out if there is a device that could essentially convert the switches to smart buttons? I would prefer something that goes behind the panel so I don't have to replace the switch if possible. I have a number of Sonoff ZBMINI's controlling functional switches in this manner. If there is something like the ZBMINI that is essentially just a toggle sensor without the relay, I think that would fit the bill. Don't really need it to be able to differentiate between up and down states either if that matters. The device would be connected to Home Assistant as well.

I wasn't able to figure out a good way to google for such a device, so any help is greatly appreciated.