Tet doesn't know
There are several references to the Tet going "offline" for periods, probably due to its orbital position.
"Alright, Tet's coming online in 30 seconds."
"Tet's offline in 15 minutes."
"Tet will be online soon..."
"With Tet offline, we need our eyes on this."
The drones are more or less autonomous and Jack's role is to maintain them. Any scanning seems to be done by the outpost and that is intermittent at best.
The drones have beacons and can't be located without them..
Without a beacon, it's like finding a needle in a haystack.
This shows that the Tet's ability to know what's going on it limited at best..even when 'online'.
It's clear that Jack takes advantage of these offline periods to go to the retreat.
He's not trackable by the outpost either. Vika says....
"Jack, we should keep in contact. Just let me know where you are."
He then turns off the comms and drops off the screen.