\starttext

\BeforePar{\dontleavehmode\llap{? }}\GotoPar

\input knuth

\startnarrower

\BeforePar{\dontleavehmode\llap{? }}\GotoPar

\input knuth

\stopnarrower

\stoptext

Wolfgang

I am applying it incorrectly, or it does not work with the buffers created by \grabbufferdata:

\setupwhitespace[none]
\setupindenting[none]
\starttexdefinition unexpanded startBufTest
  \bgroup
  \dosingleempty\dostartBufTest
\stoptexdefinition
\starttexdefinition dostartBufTest [#SETUPS]
  \getrawparameters[BufTest][xx=yy,#SETUPS]
  \grabbufferdata[TestBuffer][startBufTest][stopBufTest]
\stoptexdefinition
\starttexdefinition stopBufTest
  \startnarrower[left,right]
  \startparagraph
    \BeforePar{\dontleavehmode\llap{? }}\GotoPar
    \inlinebuffer[TestBuffer]
  \stopparagraph
  \stopnarrower
  \egroup
\stoptexdefinition

\starttext

\startBufTest
  Buffer without blank lines.
\stopBufTest

\startBufTest

  Buffer with blank lines.

\stopBufTest

\startBufTest[key=value]

  Buffer with \type{[key=value]}.

\stopBufTest

\stoptext

Rik