I agree with Eirikur. Read/Write privs don't give you privs to view definitions of objects. You need those privs to see functions. Of course, you need those privs to see any object. If you can see other objects, it might be because the PUBLIC or whatever AD group you belong to may have been given privs on specific objects or a specific schema but people aren't normally so detail oriented on privs.

If you can see other objects, it may be that the function doesn't actually exist in the given database. There could be a synonym. Search for the name of the function in the base_object_name of sys.synonyms.

Of course, it may also be that someone is mistaken and that it's not a function but a stored procedure, view, or even computed column.