It's important to remember when calling in an extraction that it will notify players in the vicinity, who are also free to attempt to use your extraction for themselves. This wouldn't be a problem if an extraction didn't hold a maximum of four players' canisters, and so if you're hiding until the extraction is ready, you will want to camp as close to the extraction point as possible to avoid having to call in a second extraction. Generally players seem pretty cordial on the whole about not stealing your extraction, but it does happen, and can get really messy when there's a bunch of players all crowding at the helipad.