It absolutely does not ever matter where you place your base, meaning, there is no code in the game that says "base in europe = X% chance of base defense mission, base in north pole = 1% chance, etc". You may notice that the base set up in, say, South Pole, would never be attacked, but that is not because it's a "safe spot", but rather because it's located somewhere where alien activity rarely happens.
If you want to know how exactly your bases get identified and attacked, read on:
Your bases only get attacked if aliens succeed in their "Alien Retaliation" mission. This mission can only happen if you successfully attack a UFO (crash land or destroy it) and only in the area where you did that. So, for example, if your starting base is in the middle of the Russia, and you chase a UFO to Spain and crash it there, then Alien Retaliation will happen in Spain, and will never find your base (unless it somehow deviates so far from Spain that it does come near your base).
Alien Retaliation works like this:
UFO will spawn and patrol around the area, going to random waypoints, doing so several times and then going back to space again. Each 30 minutes, just like the player, aliens get a chance to spot the base near the UFO on retaliation mission. Mind shield supposedly greatly reduces the chance. If they succeed in this check at least once during the mission, then a "Base Defense" mission is scheduled, meaning a Battleship will appear and head towards your base (it will keep appearing until you let it land and win a battlescape mission against it). If you shoot down a UFO on an "Alien Retaliation" mission, then another such mission is scheduled with a larger UFO size - eventually you will see Battleships on retaliation missions coming your way! If you let it complete it's run and it doesn't spot your base - then it will not appear again (until you shoot something down again).
You could theoretically just shoot down UFOs on Alien Retaliation mission so that they never discover their base, but unfortunately your radar detection chance is far from optimal. You will usually have only 10% chance to detect a UFO each 30 minutes, unless you build additional radars which most people don't. That is, until you build a Hyperwave Decoder, but by that time you probably can also build and impenetrable defense and just laugh at Battleships coming your way. So, most likely, UFO will stay undetected for long enough time to get off several attempts at locating your base.
Therefore, what can you do in order to protect your bases?
1) Always try to assault grounded UFOs instead of crash landing them. This gives you more loot (engines stay intact and more aliens survive) and does not trigger Retaliation mission. A UFO will usually land during its mission once or twice, so immediately sending a Skyranger after it will usually result in it catching up and initiating ground assault battlescape operation. To be safe, you can send an Interceptor after it as well, and shoot it down if you're sure it's finished its last landing and will not land again.
2) Do not shoot down UFOs near your base if you're not ready to defend it. If you have a UFO near an absolutely defenseless base (like, base focused on production) - never shoot it down until it leaves to a different area. If you absolutely have to shoot down a UFO and it's hovering near your base - wait for it to move into a different area and then shoot it down there. Most of the time UFO will have to land anyway, so you can ground assult it easilly since it's so close to your base.
3) It does not matter what base the craft that shot down the UFO originated from, or what country was the target of UFO's mission, or where you first engaged the UFO, or what base is the closest to the point where the offense happened - aliens will go exactly after the area where crash landing/destruction happened. So you could shoot a UFO with a craft that started from base 1 and chased it for some time and it ended up crashing near base 2 and then aliens will discover base 2 and attack it, not base 1. Even if you followed UFO with your craft right from the moment it showed up near base 1.
As you see, this is why bases in locations that aliens rarely visit will rarely be attacked - you never shoot down anything in that place. Hawaii or any small island in a big mass of water is also "safe" because you generally don't want to shoot down UFO over water (lose loot), so you won't be attacking UFO's there.
Edited by Istrebitel, 24 September 2013 - 09:04 AM.