A missing weft is interesting to note but usually has no significant effect on the value or the beauty of the rug.
Although a finished piece may be very lovely and appear without imperfections, one must remember that due to the nature of an oriental rug, imperfections will and do exist.
To identify if your rug has a missing weft, first see if there are any perceivable gaps on the front of the pile. If your rug does have horizontal spacing, it may be due to a missing weft. To check this further, locate the area in question, hold the rug on both sides lining up with the horizontal line in question, and see if the rug gently drapes, or collapses in this area. If the rug collapses or falls heavily in this area, you have probably identified a loose area with a missing weft. If the carpet drapes without opening the gap further, the space you saw is more likely due uneven weaving.
The example posted below (face of an Indian tabriz), shows a fairly pronounced pile gap running horizontally as shown by the green arrow. This gap is due to an absent weft. After a period of time and use, the gap will be less evident and may even disappear.
Below shows the same gap under higher magnification. Notice how from the front (and running horizontally) it appears as though the rug is missing an entire row of knots.
Below is the reverse side of the same area. The green arrow is pointing to what appears to be missing knots running horizontally.
The below photograph is the same carpet in a different area. This photograph was chosen because the gap is more evident as the knots are darker colors.