Countif

Help,

I have a spread sheet at work with multiple serial numbers on it. Through the months these serial numbers have been assigned to diffrent units. However these have not been updated in the spread sheet.
What I now have is a 100% serial holdings of these units that hold these serials. My problem is instead of having to go through each serial individually I tried =countif(,)>0 ( I'd like to point out, I do put the required cells needed for the formula in there). It's seems to work fine and nightlight the cells required. However on closer inspection through the CTRL+F. When I do a sanity check, a lot of the highlighted cells are wrong.

I don't know what's causing this, but I need this to be 100% correct. Some of the serials are upto 12 in length and as short as 4. Apologies about the picture, but having problems with laptop so doing this off iPhone.

I'd also like to point out I have excel 2013 at home but 2003 at work. I'm ok at excell very new so be patient with me. All help is brilliant

These errors are really stopping me from pushing on. [ATTACH]361884