Take a flashlight and start looking in, on, around, and under EVERYTHING. Kittens can flatten themselves out and crawl under/into very small places. Check under dressers, beds, closets, sofa, chair, china hutches - anything that has a space of about an inch high to get into.
Our one litter of barn kittens mangaged to go down inside the heating duct (except 2 of them). We had just brought them inside to litter train/wean and I had them in a carrier (all scared). I told our two boys NOT to open the carrier (it was in the bathroom). The heating register grate was loose in that room. But hardheaded kids do not listen! One of them opened the carrier to play with them and being scared they took off and knocked the grate off - 2 got scared and ran the other way - the other 4 went down in the duct. You could see them but they would not come to you.
SOOOOOOOOOO my husband and son had to get under the crawl space and "scare" them back out - one of us had to try and catch them if they came by. Ended up that they had to take apart part of the duct in the middle and grab them and pass them over to me.
Not a fun day - I chewed the boys out for not listening to me - BTW they were teens so its not like they were 4 or 5 yr old kids