See we have already had this problem once in Nov. My Abby baby got a hold of a ribbon that was under the heater in our new apartment (we thought we cleaned well enought but I guess not) She had to have surgery b/c it blocked her intestines. Then while at the vet she caught dystemper ( she ddint get all of her kitten shots, we didn't know) So a month and $1000 laters she ended up being ok. WE cleaned to whole house while she was sick. WE get rid of anything with string or elastic, even cut the tails off of mice and such.
But, this rubber band thing was different. It had been on my wrist at work and when I cane home I put it under a box on my dresser and shut the door intending to throw it out later (no trash in there). WEll the cats bumped the door open ( not sure how) and Abby dug the rubber band out from under the box. Hopefully this turns out better

PS- Still not rubber in the litter box but she is playing and eating and not vomiting so I am still on litter patrol.