US Tagging Errors

I don't know if I was lucky or not, but looking through a large amount of US kiloware with my Shortwave UV Lamp helped me to identify a large amount of tagged stamps that I couldn't find before. However, I also came upon the following Scott Numbers, all identified as "Tagging Omitted". All of mine are soaked and are used, with my 2011 Scott Specialized identifying the prices as follows:

Scott, Mint, Used
1327a, $200.00, <no characters>
1433a, "--", <no characters>
1461a, $7.50, <no characters>
2112c, "--", "--"

My questions are:

1. Can the tagging disappear completely from a US stamp is soaked/treated?
2. Does the <no characters> indication in Scott indicate that it is not known to exist, or just not enough information available?

I can post some stamp pictures, but since the topic is "tagging" it may be difficult to show them very well.

Yes, on the earlier stamps in which the taggant was applied after the printing (i.e., not pre-phosphored), it can happen that tagging can be completely removed by oversoaking in warm water that is not very pure (e.g., contains dilute bleaching agents). It's not guaranteed, but it does happen. Off cover used tagging omitted errors are difficult to prove. On cover is better, mint is definitely preferred. On cover examples are typically hand or pen cancelled or completely uncancelled because they got kicked out of the sorting machinery. If I remember correctly, that's how the 19c fishboat untagged (relatively common) was discovered.

I've got a few used "almost completely missing" tagging examples, but you can see a few glowing specs under filtered higher wattage UV. I think I've only been able to find about 2 examples used where I cannot see any glow -- but as I mentioned, it's really hard to be assured.

I've noticed that on examples with tagging mostly missing, the paper often glows unevenly on one or more sides under longwave UV excitation, or does not match the glow of known paper varieties -- suggesting that the paper surface has had some chemical interactions. At least one of my "no trace" used stamps also has uneven paper glow and the paper does not appear "fresh", so I pretty much assume that is not a genuine tagging omitted error.

Obviously, tagging omitted errors are a lot easier to confirm on mint stamps. They really are not that common for most stamps (a few exceptions). But the demand is quite low so the catalog values will never reflect true scarcity.

KHJ, thanks for your reply. I will be fly-specking to look for those specs or that uneven paper quality that you mentioned.

As khj has posted, "tagging omitted" is trickier to identify on used stamps. I think I do have 3 such used stamps ... one of them (C70a) was not listed in Scott so I had it successfully certified by APEX. I subsequently e-mailed a copy of the cert to Scott & it now appears in the 2014 edition.
The "--" in the catalogue value column indicates that the item is known to exist but that insufficient info was available to establish a value. You will occasionally find issues where the "--" appears only in the mint or the used column but not both. My C70a is one such item ... it was not known to exist in mint condition when the catalogue was published.
Besides accidentally removing taggant, one can also accidentally transfer taggant from one stamp to another. I have a plate block of Scott 1289 (George C. Marshall) ... the untagged variety. Except one stamp does appear to be partially tagged. Unfortunately, the tagged area looks exactly like what it almost certainly is ... a human thumbprint.