My cat may have gone

[ClairT]

I have 2 9 month old kittens, they were both castrated and microchipped 2 weeks ago, Boxer the more placid of the two began to become very sheepish this last week and when I came home from work 2 nights ago we found hew had been sick on our bed - it was food and water that hadnt been digested, i thought maybe he was just abit ill then began to get very worried when he was sick twice more but a thick creamy froth with an awfully strong smell so we took him to a 24 hour vet, they kept him in gave him antibiotics took bloods but couldnt figure out why - they said it was like he was a 'toxic' cat but couldnt find any traces of any substances, any way we picked him up last night,wish some medicationa ndw as told not to let him out for two days, his behaviour was erratic and agressive topwards our other cat - this i put down to the meds he was on, so we left him in the lounge with litter tray and food for the night , in he night my other cat who was in our bedroom was sick on the floor the same sort of creamy substance but is fine? normal behaviour running about eating drinking cuddling etc?(Pongo) I woke up this morning and the cat has gone. im putting it down to my grandma who has dementia and sever memory loss. im not letting pongo out incase he was sick because they have eaten something outside.
what should I do? what could be wrong with them? i dont know whether I should be keeping them both in because they have a virus or because they could be eating something outside? what could they be eating and how do i stop them fro doing so?

confused and stressed with my final exams in 6 days!

[Red-Mouse]

You really should be keeping them in for a while to keep an eye on them, and you should always keep your cats in at night. Cats are far more likely to get hit by cars or attacked by feral cats at night and should never be allowed out after dark.

Are you sure your cat isn't hiding or trapped somewhere in the house? When cats are ill they will often find small dark places to hide. Check everywhere and if you find him then get him back to the vets.

[ClairT]

They were in all night.. the only way i can possibly think that they could of got out was at 6/7 am this morning my nan got the milk in, he could have darted out. anyway he came home in the evening and seemed to be doing fine until this morning.. taken him back to vets for yet more antibiotics/shots etc