Results may vary....It may depend on the schools you are applying to....and also whether you utilized financial aid for the post-bachelor's program.
Some schools allow you to attach additional documents/letter, so you can just outright admit it and add it as an addendum to your application. Honestly explaining the situation shows integrity.
Some schools will verify through various channels all schools attended, and may not let your application move forward without having that transcript in hand.
In my case, I attended University of Phoenix for 1 class...and had totally forgotten about it, as it didn't apply to bachelor's degree. However, when I applied to Northwestern recently, they ran some kind of program and noted I didn't specify this school on my application--and they specifically asked for the transcript. Other schools I applied to USC (and I am already in one of their masters programs), Syracuse, and UNC-Chapel Hill did not ask for it.