In one of the early chapters of Fullmetal Alchemist (chapter 8 I believe), when Edward and Alphonse go with Armstrong back to Resembool for repairs, we see Lust on the same train.
Here, her tattoo isn't covered up, unlike later in chapter 34 when she meets with Havoc on a date.
The second image is from the end of chapter 34. This is after Hawkeye runs into Barry the Chopper, and after Mustang has started his investigation into the higher ups (which is shown in the same chapter). Envy is also shown tracking down Mustang's activities in that chapter, but this is before Edward tells Breda about what he knows from Laboratory 5.
This seems to imply that she covers up her tattoo to look more "normal" in public, but if that's the case, why wouldn't she be covering up the tattoo earlier? Or is the assumption that she covers up in public not correct? (Thinking back on it, it alos might be relevant that the tattoo isn't that large - which isn't a detail I currently remember - and therefore suspicious-looking to someone who doesn't know).
EDIT: I don't think Mustang's team knows about the homunculi specifically at this point in this. Although they have detained Barry the Chopper and so know something is up with the philosopher's stones, I remember Alphonse mentioning the homunculi's existence right before the section where Lust is killed, and Mustang being shocked by the revelation (at least in Brotherhood, where he proceeded to lose control of his driving).