a) The official novelisation as a secondary source of speculation
The official novelisation of the Alien movie by Alan Dean Foster offers us a secondary source of speculation since it was written and released as the film at the time of the movie, however little is known about what of the dialogue was his own creative contribution.
b) Centauri at top amplification
Kane gives an explanation of a possible reason why they can't work out where they are because they possibly came out of Hyperspace backwards and they were seeing Centauri at top amplification which would have been the direction where they were coming from and so Earth would be behind them.
Perhaps in Alan Dean Foster's novelisation, Alpha Centauri which is known to be 4.24 light years away from Earth, was expected to be along their route.
- "Maybe the trouble's just our orientation, not that of the stars.' Kane didn't sound very convincing, even to himself. "Ships have been known to come out of hyperspace ass-backwards to their intended destinations. That could be Centauri, at top amplification. Sol might be behind us. Let's take a scan before we do any panicking.' He did not add that the system visible on the screens resembled that of Centauri about as much as it did that of Sol. (Alien, by Alan Dean Foster, p20)
c) The Outer populated rim
Lambert reveals that they were just short of Zeta II Reticuli and that they hadn't even reached the outer populated ring.
That would be slightly different from just "the outer rim" as said in the film, because now it's one that outer rim of a populated zone.
With that she stated that they were too deep in space to grab onto a navigation beacon, let along a Sol traffic relay.
Whether Alan Dean Foster was lending his own interpretation to what the outer ring was or not, or if this was what he read in a version of the script that he had been given to novelise is still unknown.
- Several minutes of intense searching and computer co-operation produced a tight grin of satisfaction on her face . " Found it.... and us. We're just short of Zeta II Reticuli. We haven't even reached the outer populated ring yet. Too deep to grab onto a navigation beacon, let alone a Sol traffic relay." (Alien, by Alan Dean Foster, p220)