Hello @Mo0
I agree definitely a police matter.
Airbnb won't take action in cases where you are unsure about which guest has taken the money/damaged goods/behaved badly and in any case I don't believe they cover cash.
I have to say I wouldn't be leaving valuables including cash lying around when I have guests. Should always be kept in a locked room.
All you can do is speak to each guest in a calm manner (you need to be a 100 per cent it wasn't mislaid) and say unfortunately some money has disappeared from your wife's purse and that you will need to contact the police.
A difficult one, as you wouldn't want anyone in your house who is stealing from you.