Unheralded:(adj) Occurring without advance notice; unexpected
https://www.merriam-webster.com/dictionary/unheralded
I encountered the word in the New York times magazine, The Way We Live Now:11-11-01; Lost and Found.”There are unheralded tipping points, a certain number of times that we will unlock the front door of an apartment.”(Whitehead) The sentence before this gives a context clue because it says “no idea” which relates to the word unaware or unexpected. Thats how I found a better understanding for the word.
The horse pulled an unheralded stop during the horseback riding
You have the right definition of the word, but tyour example doesn’t quite get the right use of the word. It would be more helpful to explain it in the context of Whitehead’s sentence. A horse doesn’t usually announce its stopping, but you expect a little more warning when someone leaves an apartment, or a business closes. The word suggests there should be more ceremony or formal notice before these things happen, but there isn’t. What’s the next sentence? That will give you more of a sense of how he feels about these unheralded events.