This is not the full answer but it should get you started:
Define a global text field, gBarcode and put it on a field somewhere. Run this script to search your table for records with that barcode:
Go to Layout ["BarcodeLayout" ( globalsTable ) ] ---> go to the layout where you have placed gBarcode.
Insert From Device [---> select the options needed for bar code scans and specify gBarcode as the field into which to insert the scanned text.
Go to Layout [Select layout that you plan to use to show your find results here]
Enter Find Mode  ---> clear pause check box
Set Field [YourTable::barcode ; Globals::gBarcode ]
Set Error Capture [on]
Perform Find 
If [ Not Get ( FoundCount ) // no records found ]
Do what you want when no item with this bar code is found
Note: there's a bug that has been identified in Insert From Device when using the camera to take pictures and to scan bar codes at the same time:
For More Information on the bug, see: Insert from Device(Bar Code) doesn't scan after Inserting from Camera
This is one of many acknowledged bugs that can be found in the Known Buglist for Filemaker GO thread here in the Report an Issue section of the forum.
It can also be downloaded as a database file from: https://www.dropbox.com/s/jt09b82i0xijbu3/FMP%20Bugs.zip
Thank you very much. This worked very well!
I now need to trigger a different script after text has been entered in a global field and the enter/return key is pressed. how do I set a trigger in this way?
The enter and return keys are two different keys, but you can set either or both to exit the field when pressed if you set that up in the Inspector's behavior settings on the Data tab.
Then the OnObjectExit trigger can be used to perform the script.
The only problem with this is I need to be able to move between fields on this layout without running the script. if I set it up the way you have described if I click or tab to another field it runs the script. Is there a way I can set it to trigger a script specifically when enter is pressed within that field?
OnObjectKeystroke can perform a script.
Get ( TriggerKeystroke ) can return the character pressed.
Exit Sript [False] can keep the key that was pressed from being entered into the field.
Thanks for your help!