If you find yourself in the situation where your flock/herd records are out of sync with reality (e.g. several dead ewes after lambing), here's a quick way to get straight again:

1. Scan all the live ewes into a new group (called, say, 'current ewes'). Then use 'group action' to add a colour flag of your choice to this group (this is a backup measure!).

Now you can use the 'Not Scanned Between Dates' with 'Female' and 'Sheep' filters to show all animals not scanned in the animal list! 
As a double check, these animals should also NOT have a coloured flag. 

You can then select all (long press on animal screen) and add all the unscanned aniamls to a new group and do a 'Group Action' to mark them all as dead or archived as required.