Jack has been going through the same thing the last week and a half. I left them in the care of a petsitter for one day, came home and discovered Jack sneezing like mad, still congested, though not sneezing as much. JJ was sneezing too, but not nearly as bad as Jack was. Another possibility, since I wasn't the cause of the sneezing in htis apartment, is that there's a perfume or erven residual cigarette smoke. I talked to the lady in charge of the petsitter company and she said the woman who visited my cats that day didn't wear perfume, but possibly smokes.
You, yourself, may not smoke, but maybe you're around smokers... and maybe that adopter who returned the kitty was around smokers... My apartment is smoke free. Period. I don't smoke and although I have friends who do, they take it outside... all the way down 7 floors and out the front or back door. my vet told me to get some benadryl... give him half a childs dose. I haven't gone anywhere where there's benadryl in the last few days.... so today, I'm going to venture over to Target... I've been good ALL month...
In all seriousness... kitties noses are more sensitive than ours, so they'll smell things we don't. I use perfume and dye free detergent... I almost never wear perfume... the only scents coming off of me are basic things... and they don't bother with those (except when I'm just out of the shower and JJ just HAS to sniff my hair... she's bizarre anyway).
Cigarrette smoke is potent stuff... so it's entirely possible that this foster kitty is allergic to something along those lines... perfumes, smoke, pollens, mold/mildew (I, myself, am allergic to those, so I have to keep my place free of it best I can... which isn't easy)... lots of stuff like that can set off a kitties allergies.
Just a thought... since i'm dealing with it myself up here in Chicagoland. Doc told me to open the windows and get some fresh air... HA! Only if theh temps hit 45 or above.