You can use Second Wind outside of combat

The key to answer your question is if you can take actions or bonus actions outside of combat. You can and therefore you can also use Second Wind outside of combat.

I'll quote from my own answer there to explain why:

Combat is just a construct to help gameplay zoom in when a lot of things are happening at the same time. The PHB expains this on page 189:

Combat in D&D can be chaotic, deadly, and thrilling. This chapter provides the rules you need for your characters and monsters to engage in combat. (...) The game organizes the chaos of combat into a cycle of rounds and turns.

There is the proof by reductio ad absurdum: if you could not take actions outside of combat, outside combat you could not cast cure wounds or any other spell. Or drink a potion. Or remove something from your backpack. Or Run. Or Hide. Or Search for secret doors. Or help someone. And so on.

So in general, you can do anything you can do in combat also out of combat. Except for actual fighting.

You can use Second Wind outside of combat

The key to answer your question is if you can take actions or bonus actions outside of combat. You can take these only on your turn, too. If you can take them outside of combat, then you also are not limited to do anything else you could only do "on your turn" outside of combat.

You can take any action (other than actual fighting) outside of combat, and therefore you can also use Second Wind outside of combat.

I'll quote from my own answer there to explain why:

Combat is just a construct to help gameplay zoom in when a lot of things are happening at the same time. The PHB expains this on page 189:

Combat in D&D can be chaotic, deadly, and thrilling. This chapter provides the rules you need for your characters and monsters to engage in combat. (...) The game organizes the chaos of combat into a cycle of rounds and turns.

There is the proof by reductio ad absurdum: if you could not take actions outside of combat, outside combat you could not cast cure wounds or any other spell. Or drink a potion. Or remove something from your backpack. Or Run. Or Hide. Or Search for secret doors. Or help someone. And so on.

So in general, you can do anything you can do in combat also out of combat. Except for actual fighting.

You can use second wind outside of combat

The key to answer your question is if you can take actions or bonus actions outside of combat. You can and therefore you can also use Second Wind outside of combat.

I'll quote from my own answer there to explain why:

Combat is just a construct to help gameplay zoom in when a lot of things are happening at the same time. The PHB expains this on page 189:

Combat in D&D can be chaotic, deadly, and thrilling. This chapter provides the rules you need for your characters and monsters to engage in combat. (...) The game organizes the chaos of combat into a cycle of rounds and turns.

There is the proof by reductio ad absurdum: if you could not take actions outside of combat, outside combat you could not cast cure wounds or any other spell. Or drink a potion. Or remove something from your backpack. Or Run. Or Hide. Or Search for secret doors. Or help someone. And so on.

So in general, you can do anything you can do in combat also out of combat. Except for actual fighting.

You can use Second Wind outside of combat

The key to answer your question is if you can take actions or bonus actions outside of combat. You can and therefore you can also use Second Wind outside of combat.

I'll quote from my own answer there to explain why:

Combat is just a construct to help gameplay zoom in when a lot of things are happening at the same time. The PHB expains this on page 189:

Combat in D&D can be chaotic, deadly, and thrilling. This chapter provides the rules you need for your characters and monsters to engage in combat. (...) The game organizes the chaos of combat into a cycle of rounds and turns.

There is the proof by reductio ad absurdum: if you could not take actions outside of combat, outside combat you could not cast cure wounds or any other spell. Or drink a potion. Or remove something from your backpack. Or Run. Or Hide. Or Search for secret doors. Or help someone. And so on.

So in general, you can do anything you can do in combat also out of combat. Except for actual fighting.