In Digimon Tamers, we see Renamon disappear or fade out after she is no longer needed several times. (For instance, she disappears at the end of episode 7, after Rika, Henry, and Takato have rescued Guilmon.) Meanwhile, Guilmon and Terriermon do not seem capable of this: Takato has to take pains to hide Guilmon (ep. 2), and Henry has to pass off Terriermon as a stuffed animal when he is at home (ep. 1).
Why is this the case? Does it have something to do with the monsters themselves, or is it an issue of the abilities of the Tamer? (But Henry seems knowledgable enough to me, so I am not entirely convinced by that theory.)