There are several types of chat messages which can be oneboxed.
I have noticed that sometimes there are differences between searching for messages which are oneboxed an which are not.
I did not try all various types of messages which can be oneboxed - I only tried SE posts and youtube videos. I will try to illustrate some differences which I noticed.
Basically they all can be summarized by saying that: Search sometime finds messages when searching keywords which are not in the source code of the message. And sometimes it does not find messages when using something contained in the source of message.
Question. How exactly oneboxed and not oneboxed post behave when searching? Is this documented somewhere? Is it intended that the search behaves in the way shown in examples below? I believe, that if there are more features like this, it would be nice to have them collected somewhere - as opposed to stumbling on some of them randomly, like I did.)
Here are some experiments I have done in Sandbox:
Questions: I have posted the same question as oneboxed twice with longer link and with shorter link. I also posted it without oneboxing, again both with description and as a bare url. I am linking to the history, so that source of the message can be seen.
Searching for meta.stackexchange.com/questions/70417 found both oneboxed messages, despite the fact that only one of those message contains the above string.
Searching for meta.stackexchange.com/q/70417 found both messages where the question was not oneboxed. (Which is to be expected, since those messages contain this string.)
1Personally, I like that this is a possibility how to find all oneboxed posts (questions, answers). So I like that it finds these posts, despite the fact that they do not contain the word tag in the source.
2Again, this is quite a cool feature, since people are more likely to search for something which appeared in the title of the video rather than id taken from the url.