And what is the difference between an "out of game" thing and an "in-game" thing? I think you're making a distinction there that doesn't exist.
Out-of-game things are things that don't correspond to anything in the game world, and in-game things are things which
do correspond meaningfully to things within the game world. The major distinction is that characters
know about in-game things, and can discuss them or plan around their use.
Rolling initiative is just something that we, the players, do when we start to care about tracking the passage of time in six-second increments. To the characters, they're always observing everything in real-time. "In-combat" cannot be a mandatory condition to use this ability, because it's a meaningless statement.
Things that have meaning to the character might include "been the target of an attack within the last turn", or "while suffering from the Frightened condition", or even "within one round of attacking or being attack by a creature whose CR is no less than your level -2". Even though characters don't talk to each other about "turns" or "conditions" or "CR", these things each correspond to something that the characters can
understand.
They're also inappropriate conditions for triggering Second Wind, because they don't do what you want them to do. If the fighter wants to heal, but has no hit dice left, then you've just encouraged her to go pick a fight so she can get her Second Wind. Which is ridiculous, of course.
Moreover, you've just declared that any character who
doesn't jump through these hoops you've set up in order to use this ability, is at an increased risk of death in a subsequent encounter. And as a game designer, you never want the "smart" action to be something that you find ridiculous; you should avoid setting up the rules to encourage that sort of gameplay.