Some areas of the map could be designated as "dead zones" where marine communications could not reach. So marines in the dead zone could not do some or all of the following:
Use their voice communicator
Hear voice communications
Type text messages
Get text messages
Get meds/ammo/cat packs
Commanders can't drop structures there
Show up on the map
See where other marines are on the map
Aliens visible to that marine would not show up on the map
For example, there could be a back entrance to one of the hives that is a dead zone. Therefore, it would not be a siege location (this would give map makers more freedom to put areas near hives without screwing up the balance of the map by making an easy siege location).
Marines could try to sneak in through there, but they would be out of contact with their team.
Mapmakers could provide a reason for the dead zone--for example, there would be a very powerful piece of equipment operating nearby that interferes with the marines' communications.
Second, there could be semi-dead zones, which would be dead as long as the marine RT wasn't powering the sector they are in. So as marines expanded their conquest, there would be fewer and fewer dead zones.
The dead zones would add tension to the game, and would fit in well with the general atmosphere.