How to determine which application, or at least when, the UAC setting was changed to disable warnings?

Question

Windows

Over the past few years, on a machine with windows pro (first 7 then 10, clean installs, all the latest updates), a strange thing happened a couple of times, the UAC settings suddenly changed to the minimum level on their own, i.e. does not ask for any confirmation (I immediately set the maximum level after installation).
By car, I’m the only one setting up and installing applications (I confess, I’m sitting under a user in the Administrators group, i.e. UAC doesn’t ask for a password, only confirmation), the network is behind a router, there are a couple of dozen applications, and at least not all from official sources, but LONG LONG new I didn’t install or update anything, and UAC turned off at all while I was sitting in the browser (firefox and google chrome, minimum plugins). There is no antivirus (of course, there is a regular defender, but 'cloud threat protection' is disabled in it), drweb cureit/kasperski vrt did not find anything suspicious, which of course does not prove anything, but such a check has never been superfluous.
Something similar happened recently with a tablet based on win10 home, there are generally applications with a gulkin nose (far, browser, mpc player, ..).
Hence the questions - how did this happen, why does one of the most important protective barriers suddenly refuse to be turned off and how to find out when and how this happened in order to exclude this in the future?

Answer 1

Read Windows logs.