This is because while they may display buried units, these units are also cloaked. You'll have to allow it to see cloaked units.
I'm too lazy to try, but if you check out every Zerg (burrowed) unit flags, you'll see they're all buried AND cloaked. I don't know if such a unit is still cloaked if you only uncheck the cloak flag.
The sensor tower will not display burrowed units, even if you allow it in the 'Modification +' Field.
Steps:
Behaviors -> Sensor Tower - Radar -> Modification + -> Radar Filters -> Buried = Allowed
Blizzard bug report
@ajdun: Go
This is because while they may display buried units, these units are also cloaked. You'll have to allow it to see cloaked units.
I'm too lazy to try, but if you check out every Zerg (burrowed) unit flags, you'll see they're all buried AND cloaked. I don't know if such a unit is still cloaked if you only uncheck the cloak flag.
@XGDragon: Go
Detect cloaked is also allowed, I've tried every configuration I could think of to get it to show them on the minimap, but it would not work.
I've been trying to find the link between the sensor tower and it's detected icons, with no luck.