I picked this subforum because the question is best answered by someone who habitually fine-tooth-combs their own logs.
What I already know:
If you have URLs ending in a / slash, search engines will periodically ask for two things:
-- the same URL with added "index.html" (never "index.htm" or "index.php")
-- the same URL minus the final slash
The first happens even when the specific URL has always had an index redirect. Both seem to be what is known in the real world as Entrapment, but never mind that.
I know this part from direct personal observation. (Moved sites near the end of last year, and consequently disabled some log-processing functions that would normally go straight to ignore-and-delete.) It applies to, at least, G### and We Try Harder. The European search engines (seznam, yandex, etc) don't seem to bother.
What I don't know:
Now, suppose you have extensionless URLs that have never existed in any other form. (I do not have anything like this, and my test site is roboted-out, so first-hand experimenting is out.) Do search engines periodically ask for
-- same URL with added / slash
-- same URL with added ".html"
?
