Where could this vomit odor be coming from?

Late yesterday morning as I was unpacking groceries, I began to smell a faint odor of vomit which grew progressively worse during the day. It seems more concentrated in the kitchen area but the odor is wafting into the living room. I do not believe we've left any stone unturned hunting for the source of this disgusting smell. We have cats. Our litter boxes are cleaned several times daily, the food bowels are washed as soon as they are done eating and the dry food and water bowls are washed with soap and water twice a day. There is no cat vomit anywhere to be found. I sniffed around the drain because I know I dumped some bad milk down the other day, but that wasn't it - though we poured bleach down just to be sure something wasn't trapped that could be causing the odor. Refrigerator was checked, anything questionable tossed, and our freezer is getting repaired today so it's been empty for the past few days. We mopped with Pine Sol, wiped the walls down, sniffed everything to the point of where my nose is numb, and this morning it was just as bad as it was last night. WTF? Any ideas? This is an apartment, so there's no basement or anywhere water would collect - we even cleaned out the water container in back of the refrigerator recently when we realized something was wrong with the freezer last weekend. Any suggestions on what else to check would be appreciated. Thank you!