In a cavern a mile under San Francisco.
You could speculate that he had real role models for the first time after joining the Ent-D. The ship was supposed to be the best of the best, and that situation would be a life altering time for anyone (even in our universe).
Or there is the time Q gave him emotion which surely Data saved and eventually built algorithms upon later, or the time Lore transmitted him emotion sub-routines as well. Or when Bashir or LaForge hooked up any of many random unknown devices to Data's positronic net? And we'll just overlook the emotion chip Soong eventually introduced him to, right?
Then again, Data was pretty human during the very first episode. I mean, he lied to Riker immediately.
RIKER: Then your rank of Lieutenant Commander is honorary?
DATA: No, sir. Starfleet class of '78; honours in probability mechanics and exobiology.
Perhaps it was simply Admiral McCoy implying androids are bad, Vulcans are worse, therefore Data should be more human. What we see could just be Data attempting to follow an implicit order from a superior officer, for 7+ years. The whistling scene I quoted from is the next time we see Data after he and McCoy spend several minutes alone in a shuttle. Who knows how much Bones could have corrupted him in that short time.