How to search a recently read mail

[F . Durand]

Hello,

I've switched to gnus like 3 months ago and I still did not figure out 
how to quickly search for an old mail I've read some days ago.

I'm in the summary window most of the time and use M-g to refresh it.
At a moment I need to find a past mail from a keyword.
Something from
http://www.gnu.org/software/emacs/manual/html_node/gnus/Searching-for-
Articles.html
would be perfect, but it only searches into mails displayed in the summary 
view, i.e. not in already read mail.

I could use /o to first load some old mail, but how many ? If I take not
enough, I have to add them like 50 by 50. If I take too much, I have to 
wait some very long seconds to display lots of mail while the article I 
search may be in the 50 latest read mails.

I could use nnir but I'll have to break my flow by going to the group view,
then searching my entire mailbox for 30 very long seconds (ewith an indexed 
mailbox) eventhough I'm interested only in the latest one.

So do you guys know how to do that ? Or am i the only one searching into 
recently read mail ?

Thanks for your help!