My colleague didn't know this shortcut today.
So it could be a novelty for some more users...
Since some time Notes has a real cool (maybe for other mail readers not noteworthy ;-)) feature:
The start-end multiselection (select mail to start with, hold shift key and select the last mail) to select a couple of mails to move around, delete or check the read mark on them.
For the last thing, the read mark, there is key shortcut: In German version Einfügen/Insert...
But this might behave somewhat strange and I've an explanation for this...
First the shortcut key is bound to two different menu entries.
1. Mark selected mails for read
2. Mark selected mails for not read
The decision what will be done by the key is made by the status of the mail selected in the box.
So if you check mark only unread mails, but the selection is on a read mail, then the first key press marks all of these mails as unread (only changing one mail status).
There has to be a 2nd key press to mark them read.
Can be a logic program action, if the user knows that not only check marked mails, but also the selected mail will be changed.
Maybe the weirdness is only that the selection is not on the last check mark.
I'll sleep better this night knowing that it's not a bug but a feature made by Notes-people ;-)