Kilgore and others suggested a better system before.
When you get Military Base or other big SE - that it sounds a public alarm once you activate the doors or w/e and appears like a flare. Better get your shit together to raid that - because other players will see it too.
I do want to see this kind of thing though, because I'd like to see more traps - and traps matter more. Would be awesome laying a minefield in the spawn point, after your team is in and before you activate the encounter LOL