>Must have missed something:
>
>
>I have this test routine :
>
>

>       public static void Test_EXP_0071TAttached()
>        {
>            using (var context = new PluczExp_PluczContext(Config.model.SqlServer))
>            {
>                var sql = "dbo.Exp_0071TAttached";
>                var attached = context.Database.SqlQuery<string>(sql).ToString();
>                Console.WriteLine($"EXP_0071TAttached returns {attached}");
>            }
>        }
>

>
>... that returns :" EXP_0071TAttached returns dbo.Exp_0071TAttached."
>
>Here's the scalar returning function :
>
>

>USE [Exp_Plucz]
>GO
>SET ANSI_NULLS ON
>GO
>SET QUOTED_IDENTIFIER ON
>GO
>ALTER FUNCTION [dbo].[Exp_0071TAttached]()
>RETURNS int
>AS
>begin
>	declare @result bit = 0
>	select @result = CAST(
>		CASE WHEN DB_Id('EXP_0071T') is not null THEN 1
>		ELSE 0
>		END
>	AS BIT)
>	RETURN @RESULT 
>END
>

You declared that the function returns integer but you return bit instead.

RETURNS int

declare @result bit = 0