It is possible to create limits on custom fields which you don't have
global rights on, only role rights (via a queue, for instance).  Due to
the lack of context object when loading CFs in a search context (as
there is no clear queue/ticket to use), a simple ->Load returns an
object which the current user has no rights on.  This causes warnings
when attempting to inspect properties of the CF to determine how to
build the query.

As $cf never escapes beyond _LimitCustomField and _CustomFieldJoin, and
is only used to better be able to build optimal queries, simply load as
the system user.  This does not impact the results returned, but merely
allows more optimal queries to be generated.

The other possibility would be to switch to calling ->__Value() for all
accesses, to skip access control.  However, this is complicated by calls
to non-column methods such as ->SingleValue; as such, loading as the
system user was deemed a cleaner solution.