That I am not sure, you would have to read the download since often the creator will say if it will spawn or not. Most times not. If they are currently wearing it I believe it will still show. You might want to grab those townies and do gussy up on them.

I'm now pretty sure it was this sci-fi space rocket:

At first I thought it was a thermometer, but I can't find one in the health topics pictures. Yes the pictures are in the in-game interests panel, but there's no thermometer there!
I thought the red fins were the bulb of the thermometer. But thermometers filled with red-dyed alcohol are really weather thermometers or house thermometers. Until very recently clinical thermometers were always mercury-filled. Now they think mercury is too dangerous for us so they've been replaced by battery-powered electronic ones. But when you go to use it, you find the battery is flat, and it doesn't work at all! I'll warrant they still have mercury ones in Veronaville -- they run things better there!

So it was the joke about the equation, the space rocket and the elephant! "I'm sorry Sofia, but I still don't get it." I suppose I'm not the one she told it to. There's no point in asking her to explain it - explaining a joke invariably spoils it. Maybe it's an "in joke". And Sammy was in on it, but I'm not.

There's a list here: https://sims.fandom.com/wiki/Interest#The_Sims_2
Can't find one for a thermometer in there, but if it was a thermometer (and not the rocket, or the rocket in addition) there's a chance it's not tied to interests. Could be hot weather or disease or some such?

Even if mercury thermometers are rarely in use these days, they're still a much used icon for temperature.