Sick Cat? Vomit/Diarrhea in 4 rooms

[Savage4883]

I've got three cats, all under two years.
I fed them this morning as usual, and left for a family party. Nothing was out of the usual when I left. When I came home this evening, there was vomit/poop in 4 rooms. The dining room had poop in 2 locations, and vomit in a third. The poop is also in the laundry room & bathroom. The vomit is in the bathroom & bedroom.

The vomit looks like spit, white and foamy (kinda like the foam on the ocean/lake shores) Does not look like food at all.
The diarrhea is in 3 different consistencies. One is grainy, like wet sand. The other is semi-solid, but around it is literally a brown puddle. Completely liquid.
I have found 2 of my cats, they seem healthy enough. The third is still hiding and I have a hunch he is the one responsible.
The only thing I noticed different about the house was that the toilet seat was up, and I always leave it down, but my father had used the bathroom when he visited right before I left this morning, and he forgets to put it down.
Could one of them have gotten into the toilet? Could that have made them sick? Is there anything else that could cause this?
They have a scheduled vet appointment on tuesday, so we are going to bring this up with them.

Any advice is appreciated

[Marybelle]

You're going to want to find the missing cat soon, and check hydration levels (same way you do with a rat, lift the skin at the neck, and see how quickly it goes down).  With that much vomit and liquidy diarrhea, dehydration is a huge problem, especially in cats, who usually don't drink enough to begin with.  Do you treat your toilets with any sort of chemicals that stay in the water?  If not, toilet water isn't usually enough to make them that sick.  Foamy vomit can be a sign of intestinal blockage, although, accompanied with that much diarrhea, it certainly doesn't seem highly likely.  I'm sure there are other things that cause foamy vomit.  You might also want to see about getting your cat in as soon as you can tomorrow, instead of waiting until Tuesday. 

[gracegrecia]

It could be food poisoning. Call the vet.