Some years ago I looked up to the blue sky, around 2 PM, and I saw, for 2 or 3 seconds, something like a star fading out. Could it be a Supernova or another phenomena? Is it highly possible?
Several satellites (but most notably Iridium) have large reflective panels. If aligned with the sun properly, it can shine with sufficient brightness to be visible in the daytime.
An Iridium Flare usually lasts only a few seconds. It sounds quite consistent with your description.
It can be hard to tell in a blue sky with no nearby references, but an Iridium satellite would also be moving while fading out.
There are sites that can show you upcoming flares for your area. If you knew the exact date in the past, they could probably even "post-dict" what flares were available then and see if it matched.
I saw a similar "ufo" several decades ago, before there were Iridium satellites. Possibly it was another type of satellite with large reflective panels.
Thinking it over, I later decided that if an airplane reflected the sunlight directly toward me at one moment of a turn, it might make a similar flash.