Hmm, I did a quick test and it seems the following works over here:
xxxxxxx:xxxxxx
kuid:xxxxxxx:xxxxxx
<kuid:xxxxxxx:xxxxxx>
What doesn't work:
x:xxxxxx:xxxxxx:x
kuid2:xxxxxxx:xxxxxxx:x
<kuid2:xxxxxxx:xxxxxx:x>
It seems the whole kuid2 numbering system is left out of the search logic.
On a side note, if you haven't discovered this yourself, typing a '#' followed by the userID number in the Author filter will work as well as a name.
Greetings from rainy Amsterdam,
Jan