Change query to:
SELECT * FROM entradas a LEFT OUTER JOIN saidas b
ON a.data = b.data AND a.hora=b.hora and a.sector = b.sector
WHERE id_empresa = @tabempresa;
NAMED:
SELECT a.id_veiculo, a.id_empresa, a.nome_condutor, a.empresa_visitante, a.empresa_visitar, a.visitado, a.ncartao, a.data, a.hora, a.obs,a.sector, b.data as Data1, b.hora as hora1 FROM entradas a LEFT OUTER JOIN saidas b
ON a.data = b.data AND a.hora=b.hora and a.sector = b.sector
WHERE id_empresa = @tabempresa;