It's not unusual to find a few nebula beacons in non-nebula sectors. These apparently delay the rebel fleet's advance when you visit them. It seems like they always end up clustered right around where I start in the sector, though; do they still delay the rebel fleet even if the rebel fleet hasn't (visibly) arrived in the sector yet?
Alternately, how many jumps does it normally take before the rebel fleet appears in the sector?