Did you by any chance still have a scouting unit roaming the map? If they come across a still existing ruin, they could discover a technology. I assume it always gives you an unresearched-but-availabletechnology.
All other sources of free technologies have you choose which technology you want (including via espionage), as far as I can remember.
Small update
As far as I remember, city states only give you science, not specific technologies. So it goes to the research you currently selected, instead of giving you random things over time.
manpreet![Tuteehub forum best answer]()
Best Answer
2 years ago
Yesterday, while playing Civilization V: Brave New World game, I suddenly researched Refrigeration Technology, even though I was researching Atomic Theory (had 5 turns left on it).
How could this have happened?
I'm sure it wasn't from esponiage as I do not have any spies trying to steal technology from other civilizations.
Come to think of it, can City-States gift free technologies?
(I have all the policies in the Patreonage tree)