Thanks for the replies re: Mayo/Dairy.
I know that ketchup & mustard don't have to be refrigerated right away after opening
(but if weeks go by then it's best to keep them cold).
I knew about the eggs in Europe (I still wish we treated our eggs here),
but I did look up why we put mayo in the fridge.
https://www.myrecipes.com/extracrispy/mayo-unrefrigerated-shelf-life
Most commercial mayonnaises, like Hellman's Real Mayonnaise or
Sir Kensignton's Mayonnaise, are shelf-stable before opening. That's why the bottle or jar can sit unrefrigerated in the supermarket. But
once you open it up, you've got to store mayo in the fridge...
The perishable nature of mayonnaise is also why you should throw out mayo that's been left out unrefrigerated overnight. It could be totally fine—until you get food poisoning. And, in general,
the FDA recommends tossing perishable foods, including mayo, that have been left out at room temperature for two or more hours.
According to the US Food and Drug Administration, in the United States,
real mayonnaise needs to be no less than 65 percent vegetable oil, with vinegar, lemon juice, and some type of egg yolk.
It's the inclusion of an egg yolk that makes mayo perishable and notoriously tricky to handle. "Traditionally, mayonnaise is made with raw egg yolks, and therefore carries a slight risk of salmonella infection," explains food scientist Harold McGee in his book
On Food and Cooking: The Science and Lore of the Kitchen, adding... This raw egg is part of the reason
McGee recommends treating mayo, "as a highly perishable food that should be served immediately or kept refrigerated."
I also read elsewhere that some American restaurants buy specially made mayo that is safe to leave out
all day while they use it!