This page has to deal with the patterns in that the enemies spawn. All mission types will obey the generalized enemy spawns unless otherwise stated below. (a majority of this information is from personal experience and should be tested/edited with others experience/datamining.)
Generalized enemy spawns
It seems that the engine checks to see if there are any rooms currently uninhabited by players within a certain max/min distance. If there are no suitable spawn points the enemies will spawn regardless of player position. Enemies in general missions seems to have a mob cap as well(max and min need to be tested). This mob cap is affected by the friendly mobs in invasion missions and Nekros' warframe ability.
During an alert status of a mission(ie the alarm has been sounded and the enemy knows you're there) the enemy mob caps are raised slightly. If the alarm is shut off the caps are not reset to normal but slightly above the unalerted state. If the alarm is set off again and turned off, the new mob cap will be slightly higher than the previous "normal". It is possible to stack this, but it has not been tested/proven.
Enemies will despawn unless they are within close enough distance. If the player stops moving for a short period of time or the goes outside the range the enemies will attempt to pursue.
In survival missions the max/min mob caps are set at a constant(constant needs testing) that will increase based upon time in mission along with their level.
Defense missions are similar to those of survival, however rather than being based upon time it is based upon wave. It would also seem that in defense missions the constant is reset after every 5 waves(ie after reward has been offered) with an added amount(which will also increase per 5 waves). Enemies in defense missions will not despawn if they are around the defense point.
Mobile defense missions
Mobile defense missions use a form of waves to spawn. The mobs will continue to spawn(assuming there is proper area to) until the cap has been reached or has been close to the cap. The mobs will then cease to spawn until the mobs have been lowered closer to the min, in which time they will begin to spawn again(this will loop until the defense is completed). Once the defenses are completed or during the start of the missions before the defenses have started, the mobs will continue to spawn with relation to the generalized rules.
In invasion/exterminate missions, the max mobs are shown on the right, however the mobs have not spawned in yet until the player moves within a certain distance of the room(which obeys the generalized rules). As an extra to these missions, enemies will be prevented from spawning in rooms that the player has already entered. Both friendly and enemies will spawn in said room once the player(s) is within said distance.