Now I realize that this is ultimately unknown. But according to the Canon, The Lone wanderer left DC sometime in between the end of Fallout 3 and the beginning of Fallout 4.

So my question is why? and where?

What are your theories? What was your wanderer's reasoning for this?

I think it's up to the player. Just like what happened to all protags in Fallout, it's up to the player. Except for the Vault Dweller, who was vital to the Chosen One's story.

My Lone Wanderer headed to New Vegas to become the Courier, and then rode into the sunset, never to be seen again. Just my headcanon, and a lot of others too, going by how many people downloaded and played Tale Of Two Wastelands.

Like @Corvo said. I think it's ultimately up to the player on what happens to the protagonist of every game. As for why he left...maybe the Capital Wasteland held too many painful memories. Or maybe he felt there was nothing left for him and wanted to see the rest of the world. I can only speculate where he went. I think he just...wandered. Just traveling from place to place and occasionally doing a good deed along the way. I know alot of people like to say the Courier is the Lone Wanderer. While, I don't disagree, I don't entirely agree either. I prefer to keep them separate.

I get that. Leaving it open ended for the player (which I think is great). I've just always been curious if there was, like, a "default ending" for those who may not have had the pleasure of playing.

It's just seems to be a theme for Bethesda, or for the TES series atleast. (Like how The Champion of Cyrodiil became the next Sheogorath, or The Nervarine travelled to Akaviir.) So I was curious if there might have been some post-game canon that I wasn't familiar with.

With headcanon put aside, The Lone Wanderer is dead. Activating the purifier was their fate. Some will argue that Broken Steel changes that outcome, but the only thing that's canon from Broken Steel is the war between the Enclave and the Brotherhood. Other than that, it's fairly clear that the Lone Wanderer sacrificed themselves to complete the goal James and Catherine wanted for the people of the wasteland. I certainly wasn't a fan of Fallout 3's storyline, but it bears some sentiment when you look at it that way.

However, we're all entitled to have our own opinions and beliefs of what happens to our characters. My Lone Wanderer continues to serve the Brotherhood, but actually does end up dying alongside with Sarah. I purposely left this vague so it fit into the canon of Fallout 4 (Even though I have strong dispositions with Fallout 4's lore).

I like to think The Lone Wanderer traveled to Philadelphia, New York, or Baltimore and just continued doing what they do best.

Eh, got a job in Nevada, some douchebag named Benny shot her in the head though

The Lone Wanderer and the Tunnel Snakes went to rule New Vegas.

Went to vegas and become a courier, got shot in the head, went on to get revenge, and then used a time machine to go back in time and start a family before the war only to get frozen for 200+ years.