Ann,
There are a couple of things you could do in case it happens again...
Visit their local police station, explain situation and that she may well wander in future and leave them with a photo of your mum together with background information etc. by doing this they can immediately start looking for her if you give them a call. I would explain to your dad that he MUST phone them first as they can perhaps locate her before she perhaps gets on a bus or a train or even a taxi and is then out of the immediate area!
Secondly, make a note of your mum's name and a phone number and place a copy of it in any garment that your mum might wear to go our. Buy one of those charity bracelets and write her name...dementia...phone number on it in permanent marker . Get your mum to wear it all the time and point out it is to show support for the armed forces..you and your dad could also wear one too if it helps her keep her one on!
Your dad might need to consider keeping the doors locked too, in future!
Fiona