I tried Shantytown and there was nobody there. I think people think there are respawning enemies but there are only ones you haven't killed yet. Since I'd gone through 100% of the area, there is nobody left!
I think I'm going to have to let it go. Knowing others are in the same situation gives me comfort that I'm not missing a fix. I've combed all the levels pretty well already so I doubt I'm missing a grenade crate. I've got way too much other content to play to worry about chasing this down. It's not like it's the last thing for a Plat or anything.
While it's not the end of the world, it is kind of annoying and an example of bad design to put collectible items behind walls that require non-respawning ammo to destroy. Maybe eventually they'll patch a crate to spawn there or something. Thankfully I enjoyed searching out the other collectibles - it was fun and didn't feel like a chore so that's good.
But I'm enjoying my free copy of Sleeping Dogs way too much to put any more time into this.