Formula? to find existing use of a reference number across 12 tabs then find next avail #

This may be titled incorrectly, or even be in the wrong subforum and bear with me as this is a little more complicated to explain:

1. I have a tab per month with each month's tab identifying a number of "Need to locate" rows using a Combo Code (concat'd from I-code and Adj W/H).
2. The Sys No column is using the Combo Code to look for the same Combo Code on a Additions tab and returning the Sys No from that Additions tab
3. What I am trying to do is not replicate the Sys no once it has already been used and grab the proceeding relevant Combo code's Sys No from the additions tab
4. However, this needs to reference each month's tab to ensure we don't have any duplicated Sys No's on any of the months' tabs nor within the month's tab.
5. For instance the Sys No appears on both the month's example tab on the provided sample doc, which would not be a good number for the second occurrence.

I also have no issue combining all of the month's "Need to Locate" rows onto a single tab to make it easier.

Thank you very much for your assistance