So far, in the first couple of hideouts, I'm noticing a lot of locked doors with the triangular exclamation mark (NOT doors that require a lock grinder). It is possible to try and open them and they make a 'locked door sound'. Are all these doors possible to open some how? If so, that's good news - there's a lot about!

Nope, you can't open them. You will see A LOT more of those.

I think they are there for other routes for different missions that could take place at these hideouts.

Yes, generally those are simple doors that may or may not be open in later missions or later on in your current mission. Many times after completing an objective one of those doors may open when new enemies suddenly emerge or to route you to a quick exit. Other times, on another mission to the same location you may find some previously open doors closed, and new one open.

Edit: I should note that there a few rare instances when you can get these doors open by detonating a nearby canister or using some electrical/EMP device on a nearby "trigger".

I have found lots of these doors in the first half or third of the game. One or two have either since opened or a locked door has appeared blocking off a previous path from another mission. I have also found doors without the exclamation mark which cannot be interacted with, which makes me think the ones with the ! symbol MUST do something...? Perhaps I'm just in hoping for something to explore or for a reason to return to the game once completed.