When I understood the question right, you want Adabas to set the jobs condition code to non-zero.
With the ADADBS utility you will receive a cond code 0 in the case the file is used and also if it is not used. This is because the function itself is correctly performed. The utilities set the cond code to 4 or 8 in the case that the required function cannot be performed (cond code 8 ), or if they want to notify the user that there is a warning (cond code 4).
What I can think of would be the possibility to run ADAULD with two parameters: NUMREC and UTYPE. ADAULD does normally open a file with EXU, if there are update users then ADAULD will get a rsp-48 and this will lead to an error and to cond code 8. You can use the parameter UTYPE to check also for read users. If there is no user active you certainly do not want to have a long running ADAULD to get the file locked for other users, NUMREC=1 should minimize the runtime.
I have no other ideas so far,