Midcentury house "smell"  

Tuna casserole.
Seriously, though. If the homes have exposed cedar roof decks (low-pitched cathedral ceilings) as many around here do, and you're sensing a strong, somewhat aromatic "wood smell", it could be a symptom of active rot caused by roof leaks. Yes, western red cedar will rot and attract carpenter ants if kept wet. Don't ask how I know this.

Raw chicken.

I will relate and let you know I think it is a "sign" you should not acquire those properties. For me, should I be interested, it is an indication to call a professional cleaner. Is there anything more insignificant with these midcentury homes?

I find that a lot of plywood used in cabinets from the era has a very unpleasant smell. It is most pronounced when you open a cabinet that has been closed for a while. We had a built in linen closet that suffered from the problem, and all the sheets stored in it came out smelling a bit like sour milk.
People back then smoked a lot.

and the early laminate glues (and still many today) used some serious chemicals, with formaldehyde and urea top of that list.....neither of those chemicals smell very nice.

Add a hotter-than-normal summer to accelerate chemical reactions and resultant off-gassing, not to mention a closed-up vacant house, and... voila.
Assuming it's not just stinky wall-to-wall shag carpeting, of course.

Urea is odorless. I think you mean Urea Formaldehyde, as in glue.
Urea is also tasty. The awesomeness of the humble pretzel is down to urea.

Yes.
I happen to like the smell of an old house, but I also like the smell of gasoline. I just don't like the smell of Cheeto's and/or Nursing Homes. Foot odors and light switch covers can smell wicked, too.
carry on,
Aunt Mark

Anything that is old will have some sort of tolerable smell unless the houses you looked at has more serious issues like water damage/mold, a colony of bats living in the crawlspace or stray animals squatted in it. I have worked on several old houses and each and every one of them has a distinct smell which goes away after awhile. If the smell bothers you, look at new houses instead, they do smell too like Lowe's or Home Depot.