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.