“I found this rigid pink tube at my Airbnb. It displays 3 holes on the bottom. It has some kind of a whitish residue. It’s mostly cylindrical, but has a flat side with numbers to measure something.”
Answer: "The tube comes with another part so that you can suction-mount it to the side of your bathtub, and you’ll have a bath toy for your kid. There’s also a ring that comes with the set, and it fits under the flared edge of the tube so that your kid can poor water in with a cup. The latter also comes with the set.
The numbers measure nothing, they are just here, so the kid can practice counting. The hole in the bottom allows water to drain out back into the bathtub."
Could it be the picture is upside-down?
"Tastes like lime..." I'm more concerned that the person would taste something that they don't know what it is (the answer is probably correct but still...)