\setupbodyfont
  [dejavu]

\setuphead
  [section]
  [number=no]

\starttext

    \startsectionlevel [title=Alpha]

        See section \in[invalid].

        See section \in[sec:bar].

        See section \about[sec:bar].

        \startsectionlevel [title=Foo]
        \stopsectionlevel

        \startsectionlevel [title=Bar, reference=sec:bar]
        \stopsectionlevel

    \stopsectionlevel

\stoptext

