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).