Hi Moyock13.
Missing ink is very rarely if ever a sign of a retouch. A retouch is usually extra ink where someone has tried to manually touch up the plate by using a tool to strengthen a line or other element with some minor engraving directly on the plate. When this is not done well, lines can look too thick or they may not be straight. In the most severe cases, they may deviate from the correct path and show as spurs or as partial extra lines on the final printed stamps. These severe examples of retouches are sometimes referred to as "engraver's slips".
The missing ink from the vertical line on your stamp is more typical of plate wear or of a relief break on the transfer roll that is used to lay down the plate. In the latter case, these missing recessed areas often show up as broken lines for stamps from many positions on the plate and sometimes across several plates until a new transfer impression is put into use. They are usually very minor in nature and not of significant interest to the average collector. Where they become useful for specialists is when they can be used for complex issues like the Admirals to help determine what plate(s) a stamp comes from. Marler's classic book on the Admiral stamps uses these relief breaks to categorise each stamp into types for plating purposes. There are more than 30 major types for some values in the Admiral series. Many of them feature small breaks in the vertical lines of the value tablets as your stamp does.
Interestingly, while these types of line breaks are not characteristic of retouches, they are good examples of the type of issues that a retouch is often trying to fix. If someone tried to manually strengthen the line of your stamp and the repair showed as a much thicker line or a line that was extended into the bottom frame line, this would be an example of a typical retouch.
There are several retouches shown on my FlySpecker website. You can see a good example on an Admiral stamp at:
http://flyspecker.com/106-132.html. The second stamp shown at the above link features a nice engraver's slip where a retouch was being done to strengthen the vertical line at the top right of the stamp.