The following code:

    \setupregister[index][n=1]
    \starttext
    \index{A}\index{A+A}AA\seeindex{AA}{A+A}.
    \placeindex
    \stoptext

generates the following log entry.

    structure       > registers > invalid see entry in register 'index',
    reference 'AA'

No similar entry is produced for a non-+ target. This does not appear to 
cause a problem in the output, but the log message
is annoying, and may point to incorrect logic in the source (strc-reg.lua).

-- 
rik