What can make a smell like spoiled milk besides actual spoiled milk?

Wallpaper paste can smell like that. Do you have any upstairs? If it has gotten damp or wet, the glue puts off an odor.

That milky smell is almost certainly of bacterial origin. It may come from bedding that needs changing or a mattress that needs airing. It often arises when cold, damp air enters a recently occupied bedroom, so it tends to occur more in winter.

The following may help: Wash all your bed linen at on go, using powder, not liquid detergent. Avoid using fabric softeners for a few washes. You may need to wash the bedding more frequently. Stand the mattress upright and allow air to get to both sides while the washing is under way. Do not dry bedding on your radiators; use a tumble dryer if you have one. Store your spare dried bedding in an airing cupboard or other warm place, to keep moisture to a minimum. Leave your mattress to breathe before you re-make the bed, and turn it over once a month. Use an electric blanket during the day to keep the bedding and mattress well aired. The warmth will displace the moisture that accumulates overnight.

Eventually you will find the right combination of actions that suits your household conditions.

Do you have any animals in the home? I ask only because we had the same type of thing in our house (rotten food smell & we could not find the source). It would waft by then disappear; we also could not track the source. It finally did get stronger & I tracked it to a spot between our door & our couch. Turns out our cat got sick & threw up under one of our end tables (bottom shelf of the table was low to the floor, so we never saw it; GROSS!!!). The only thing I can suggest is to look under every piece of furniture & in every nook & cranny of the upstairs. If all else fails, you might want to check the heating ducts (could be a squirrel or mouse that crawled in & couldn't get back out). Hope this helps!

Could it be any other dairy product? Could a mouse have snuck a bit of cheese up there and it's gone bad?

Any little snacks in a jacket pocet?

You check with your local pest control. They might could tell you if the smell could be from a rodent or pest that has hid a stash of something in your wall or died or something.

Wow, that's a tough one. Good luck.

Maybe it's a dead mouse/mice. Look all over the upstairs for it & hopefully you'll find it. It may have died between the walls, in which case, you'll have to put up with the odor for a while.

Oreos without milk is like Batman without Robin! Like Romcoms without comedy! Like Justin Bieber without countless amounts of haters! It's just... Wrong.

This sounds really gross, but it might be a dead mouse. They let out an unbelievable stink and sometimes it's hard to find them.

Good luck tracking down the smell.

I can't tell you what it is but maybe what it's coming from...the carpet. I don't know what causes it but my carpet was smelling like that, too. I had to shampoo it twice before it smelled better. Good Luck!

Maybe you left an open empty yogurt container somewhere like under an end table or something.

mildew can smell like sour milk, check in areas with high moisture