Their only most recent appearance is in New Super Mario Sunshine as enemies once again.
For the most part, unless they see Mario, Swoopin' Stus will generally wander in an outward direction without any real, apparent purpose and won't create a larger 'goop area' unless they fall off a ledge and pop on impact.
Once a Swoopin' Stu spots Mario, they will turn white for a second before making a "bwooww-wop" noise and swooping at him. They appear prominently during battles with Piranha Plants spawned from Goop Generators, swooping at Mario whenever they have the chance and disrupting his own attacks on the Piranha Plant.
Swoopin' Stus can be destroyed with a blast of water from F.L.U.D.D. or by being jumped on, though the latter method will leave behind a puddle of Goop that can spawn more Swoopin' Stus. This can be avoided, however, if they are jumped on while they are in the air. Stomping on them also causes them to occasionally drop Water Bootles. Eventually, Swoopin' Stus pop even without provocation, but will not leave goop.
In Mario Kart Dimensions, Swoopin' Stus appear on the course "DS Delfino Square", where they appear all throughout the course (which has now become more challenging). They act like Bloopers do, as when they jump on a kart, they cover the screen with lava goop.