The other neons might have eaten them if they were dead already. sometimes when you buy new fish there is a difference in the water they are in from the shop, to what you have in your tank (PH ect...). a lot of fish are hardy but you might find one or two it affects more in which case you will loose some - not many, then the others will eat them. (there are some other causes but with a new tank this is probably the most likely)
I've had a lot of fish just dissapear, I've got 3 tanks and used to work in the fish department of a pet shop, and fish definately eat other dead fish (or they will attack/pick on poorly fish) I've only sometimes found a body.