1) Picking up certain objects seems to trigger hunter killers and their ilk to come hunt you down. It is no way time based. The Items I've discovered so far that seem to do it are: Weapon Tech, Officers Laptop, Neurolink. Photo of a Couple, Data Cache. Opening too many containers also seems to spawn them. Though its hard to if its a random roll or a set number. ** With the changes in the patch it seems more clear that there is a threshold of some sort; Some actions and Loot raising it more than others.

1b) Don't panic, especially now that HKs spawn farther away. Your basic HK is well armed and armoured but ultimately is just a EOD soldier with a MG34. You can outrun them, out climb them and evade. And even if you cant you almost certainly have the time to find a chokepoint. They can be stun-locked and killed just like most other infantry. Just beware that they will bring the wrath of their faction upon you so try keep away from masses of their factions soldiers.

2) Triggering a Timed Extract will almost always spawn a wave of enemies to patrol the area. Either hunker down with your friends and prepare for a fight or Run, and wait for the enemies to hopefully clear out once the countdown timer is up.

2b) The timed extract seems to stay open more or less permanently though the icon sometimes switches back on the hud.

3) The Scav Dash is a great help. It's under "Dive" in the options but it's really more of a slide. Bound to "Ctrl" by default, it can be used while crouching or standing and allows you to slide quickly left or right without making any increased noise or notability. On top of helping you get into cover it also seems to do pretty well at getting over weird floor geometry and getting unstuck from places.

4) Vendors have other pages of items to sell! Weapons venders sell attachments and gun parts, the rig vendor sells add ons and large items. Make sure to check the bar on the left and see what they have to offer. (Weapon parts are often pretty cheap so skip weapons mods crates and fit loot to buy more IMO) Turns out they have zero weight so loot away.

4b) Specific weapons parts are unlocked as you gain experience with a weapon.

4c) Customise your weapons! With these parts you can add new things to more or less any non-surplus gun. It's hard to tell how much effect many of the parts have, but at least the sights, supressors and magazines are obviously helpful.
4d) Tactical Flashlights do not give away your position in anyway, they are also only on while ADSing.

5) Dig up your grave. When you die in a map you leave behind a grave, marked with a symbol on the map and compass. Go to where you died and you can recollect any gear and equipment (Not Consumables or Loot) . It isnt stored in your pack and is simply sent back to your stash if you manage to exfil this time.

6) Dead Map. The term I have for an event that I've had happen on 3 different maps so presumably it happens on all of them.
-All/Most of the lights go out
- Thick Fog is Everywhere
-It's nighttime
-Cyborg zombies and worse are everwhere.
-Ash/snow? on the ground
It's pretty darn spooky.

7) Doors are LOUD but seemingly impenetrable*. Sure that there's no one close on your side? Just slam the door in an enemies face and they'll be stuck. Be careful though, some doors only have buttons to open on one side.
*Exo suits non-withstanding.

8) Rigs. Certain characters can use certain rigs, but despite what you might think from the skills. the characters Without the rig skills have more options, its just that the rig skill characters can unlock options to match them. Scav girl for instance can't use any of the rigs that Bundleton offers without those skills.

9) Backstabs exist; get behind a unaware soldier, and there should be a prompt of a flashing crosshair. Press the middle mouse button (or whatever your melee key is) to silently take them down.

10) Ping (Press Z) can do more than just highlight and track enemies, you can also ping items which can tell you what something is at a distance and in the dark.