Display missing sequencial numbers

I have a sheet where one column has sequencial numbers from lowest to highest with some missing. So for example: A1 1, A2 3, A3 5, A4 6. Is there a formula that will display all the numbers that are missing from this column without skipping rows? So let's say: B1 2, B2 4 ect.

Basically I will be putting this on a different tab, so someone can go to that tab and see what numbers have not been used. It is a large range and is difficult for someone to scan through it all.

Thanks for any assistance.