I've noticed quite a few values for tagged varieties in the US Specialized catalog that are missing. I'm not talking about the more rare ones, but some entries may have a value for mint but not used. Some of these don't seem to be rare.
Is the reason for this that the taggant can be washed off or altered in some way when soaking? Or perhaps there is not enough pricing data to support a solid value?
I don't know about U.S. issues, but collectors of tagged Canadian stamps generally don't collect used stamps for the reason you mentioned — the taggant on at least some of the tagged stamps is water soluble, and even migrates from tagged stamps to untagged ones that are stored with them, as in a nickel box (probably a dime or quarter box these days!).
Les, I have been working on used US stamps, and I have found similar to what Bob has described for Canadian stamps. I was checking a stamp for tagging that was hard to see. I turned off all the lights and the stamp didn't have any tagging, but my fingers sure glowed green! The taggant was even on the plastic sleeve of the card holder.
Since we are on the topic of tagging.....has anyone seen what the Luminescent ink on the 5 cent circus wagon stamp looks like? Do you use the same shortwave UV to see it? I have tons of copies and none glow under short or long wave UV.
Scott lists both 2452Bf and 2452Dg as "printed with luminescent ink". It doesn't say phosphorescent ink, so I'm not sure how to tell them from the regular ones.
Try this link for a discussion on your question. It looks like under short wave UV, the ink glows orange. With the ink color being red, that may make it difficult to see the orange.