This sounds easily 95% plausibleRegarding how it got there - I'll bet someone was replacing the flange and/or wax ring under one of the toilets and tried to sit that decorative item over the hole so it wouldn't stink up the room only to find that it was small enough to fall in. It was sitting on the toilet or somewhere nearby and looked like it would do the job. I expect that level of dipshittery is much more common than mal-intent -- you have to ask yourself if the odds of someone taking a pot/vase/urn somewhere to try to ruin their plumbing is more likely than someone ****ing up while working on something (probably after several beers and his wife bitching in his ear.)
I'll bet the wife in this story still wonders where the **** that went, and that dude still swears he doesn't know what she's talking about.
Spoken from experience of messing shit up?