User can alter procedures without permission
Do you see anything that has been granted or role membership that shouldn't exist?

EXECUTE AS USER = N'your user';
GO
SELECT * FROM sys.fn_my_permissions(NULL, NULL);
GO
REVERT;
GO

SELECT * FROM sys.database_permissions
WHERE grantee_principal_id = USER_ID(N'your user');
GO

SELECT r.name
FROM sys.database_role_members AS m
INNER JOIN sys.database_principals AS p
ON m.member_principal_id = p.principal_id
INNER JOIN sys.database_principals AS r
ON m.role_principal_id = r.principal_id
WHERE p.name = N'your user';

SELECT r.name
FROM sys.server_role_members AS m
INNER JOIN sys.server_principals AS p
ON m.member_principal_id = p.principal_id
INNER JOIN sys.server_principals AS r
ON m.role_principal_id = r.principal_id
WHERE p.name = N'your login';


It is possible these developers are in the db_owner role without your knowledge or have been granted ALL on the dbo or another schema.

Make sure you execute these statements in the right database, and also validate that your users are logging in as who they say they are logging in as. Perhaps they have the sa password and you don't know it. Finally, you should make sure that the database principal maps to the correct server-level login.

SELECT dp.name, sp.name
FROM sys.database_principals AS dp
FULL OUTER JOIN sys.server_principals AS sp
ON dp.sid = sp.sid
WHERE 
(
  dp.type &lt;&gt; 'R' 
  OR sp.type NOT IN ('C', 'K')
)
ORDER BY 
  dp.name COLLATE DATABASE_DEFAULT + sp.name COLLATE DATABASE_DEFAULT DESC;