I don't think that there was any microprint on the Sparkling Holidays issue. You can usually determine whether microprint may be present by looking at the technical details for the issue in the bi-weekly postal bulletin (available on usps.com). Look for the "Manufacturing Process" entry … occasionally you will see "Offset, Microprint". As an example, I just looked at two of the 2018 holiday stamps … Madonna & Child and Kwanzaa. Both had microprint specified. After a lengthy search, I did find a microprinted USPS on the Madonna & Child. I never did find one on the Kwanzaa however. While I think "Microprint" usually (or probably) refers to some microprinted lettering, this need not be the case. I really don't know whether "Microprint" being specified means that there will definitely be microprinted lettering on the stamp. I do however believe that without "Microprint" being specified, there will be none. Be forewarned: this is all supposition on my part … perhaps someone in the know can clarify the situation.
Just a followup to my earlier post. Returned today to the 2018 Kwanzaa stamp. Scanned it into my computer & used software to blow it up. At which point, the microprinted USPS was obvious. So I then turned my attention to another stamp for which I had been unable to locate any microprint … RW85 (Mallards). Under "Manufacturing Process", the postal bulletin had listed Microprint "FWS" for this stamp. I had guessed that I should look for a microprinted FWS (Federal Wildlife Service) as opposed to a USPS. But even after blowing this one up with computer software, I could locate no microprint whatsoever. Of course, this doesn't mean that there is no microprint. One of my first uses of the computer to magnify a stamp was for the FAS (Flag for All Seasons) issue. One of the vendor's stamps had extremely faint microprint. Even magnified, it was very difficult to locate … without using the computer, I would never have located it. But locate it I did.
Of the stamps I have examined, it is only the duck stamps for which I've been unable to locate microprint. I do not know whether the microprint is hidden as you suggest. My first inclination is to say no since there is no real need to prevent reuse of duck stamps … they can't be used for postage and are good for only a single year. But I also know that at least some of the stamps do exhibit a response under short UV, sometimes a very interesting response. E.g., parts of the duck may glow while others do not. This may be due to the ink being used … or it may be some form of creative tagging. Also, microprint can be used for other reasons than security. I recall reading that the first use of microprint on U.S. stamps was on the 1992 wildflowers issue. It was used to create part of the dot pattern on one of the stamps. So it's entirely possible that there is no microprinted message on the stamp. Or, as you suggest, it may be well hidden. Maybe someone out there knows the answer. One thing that I have learned by searching for microprint on U.S. stamps is that the computer can be a highly useful magnification tool.