Hey Gnomeface, this coexists with my theory, especially when you added the part about using just a bit of current that isn’t enough for an analog bell to trigger the “ding.” When I said “bridge the connection,” I suppose I wasn’t clear that the Eufy instructions were to use an included cable to close the circuit behind the analog bell/ringer. I imagine that running the video and audio functions (especially video and intercom) without a battery (mine is wired-only) would be enough to trigger an old-fashioned ringer (or confuse a digital one). The app for the wired version doesn’t show me an option for “use existing chime.”
AC power is definitely what this uses, the directions even state that it doesn’t matter which screw terminal you pair the existing doorbell wires to because the polarity switches anyway. Most US appliances are designed to run off of 120 V AC power (I think it’s 220V AC in the UK and most of Europe?), with a step-down transformer for low power devices like doorbells.
My house was built in 1952, and we discovered that the original transformer had died… this may have been why the front doorbell had been removed and the hole for the wires patched up (extra fun for our installation). That or the previous residents didn’t like visitors, haha… the man of the house was an electrician by trade and had converted the garage into a DIY workshop, so clearly he would have had the capability to see what was going on, and didn’t care about fixing the bad transformer. We didn’t use a plug-in transformer (do those go into a wall socket??), I had an electrician friend and we properly replaced the wired-in transformer in the basement ceiling. 
