UK Landlord Forum / Life

I'm noticing the tenants are leaving the patio doors open half way through while they are away for holidays for weeks. There is as well all windows and patio doors left open throughout the day while they are away for work as well.
There is quite a bit of mould formation now near the patio door walls and windows due to leaving the flat damp like this.

I am getting bit concerned that it may cause significant damage due to wind causing damage to windows left open like this

What can be done ?

Ask them why they're doing it.. and then, if there's no good reason, ask them not to....