Playing on PC, bought the Scorchers via Steam, it shows up under the "DLC" tab in the game properties in Steam, but none of the content is accessible. This applies to both starting a new game and continuing from an older save. The only weird thing I can think of is that I'd previously transferred the game from an older PC to a new one (and no, verifying the game cache doesn't resolve it either).

What gives?

At the Hagar settlement you don't have an entrance to the "Hagar Caves"?

Nope.

I've viewed the walkthrough, I know where the door is, but the door is just boarded up. No entrance available.

How big is your game file, do you have the DLC downloaded? There was an issue on the release day where I had the add-on on the DLC list but it wasn't downloading. Then it suddenly started working. I don't really know what you can do but it sounds like Steam hasn't downloaded it.

It's downloaded. Just deleted it and re-downloaded (via cache validate) and still the same. Further info - the console log during startup has "found DLC3 and info.txt - user does not own this DLC" but yet it apears under the DLC tab in Steam and viewing the page for it in the Steam client informs me that I already own it.

Trying a redownload of the game .exe now to see if that fixes anything.

Just noting that I fixed this by reinstalling Rage. The only difference I can find is that the "Rage" folder has it's name in all caps post-reinstall, whereas it was in lower-case following the transfer. I do hope that the DLC authorization check isn't case-sensitive...