Mostly all been said, but just to sum up (from the horse's mouth)...
Backpacks... locking is out, because that would require a mod/addon - and we don't want to use addons because they reduce the discoverability of the server.
Backpacks - we *might* possibly be able to add a noise... I've been thinking long and hard about this, and it *may* be possible to stack a second event-handler on to the I-keypress, which looks at the target of the inventory-investigation, plays a ziippp sound, then passes control on to the normal Arma3 inventory handler. Not sure yet - it's all theory. Will notify the forum if I crack it.
Holstering - no, not currently planning to do this. Too many bugs. There's nothing wrong with animations, per se, but there are issues to do with disappearing guns. The holster 'trick' is to delete the weapons from the player, mark them as being owned-but-holstered in a variable, and retrieve them later in reverse when unholstered. Trouble is, the variables sometimes do not sync properly across the client-server divide, so sometimes they never come back. It also means that cops are unable to discover them if they 'search' a player - we'd have to completely rewrite that searching code to examine the holstering variables, and that is prone to failure too (propagation of client-side variables across the server-divide to another client is always a bit 'iffy' in Arma). Worse, there are issues involved in the way Arma behaves if you remove a primary weapon (rifle) - it will automatically place the handgun into the players hand, which means they're still in breach of the 'no guns out' law. We'd effectively have to create a 'double-holster' to enable BOTH weapons to be hidden - which doubles the risk of glitching and weapon loss. We'd also have to write scripts to handle holstered (deleted by marked in variables) weapons when a player dies, respawns and disconnects, or they'd go missing.
All in all, simply not worth it, when you can merely shout at a backpack-invader, knock him out with Shift-G if necessary, and if he still persists (and you're not in town), shoot the silly bugger. OK, so you may have some RP to do with a cop later to explain that you shot a robber, but if you do it right (and you make yourself known, rather than wait to be caught), you'll probably get off with a token fine.
The binos trick is now outlawed, I believe (check local police rules - it may be different in some cities). I've also discovered today that 'backpack invaders' who use the binos trick don't show up with that tell-tale 'lean-in'... and thus backpack diving with binos is virtually indetectable. Probably another reason it'll go, tbh.
