Hi Tess
This can be quite normal, I also live a fair distance from our property and have been welcoming guests for 7 years (obviously not just through airbnb!). So for example I have a guest staying this week, she accessed her guest guide for arrival instructions etc (we use vip-guest guide which contains the arrival instructions etc). I also emailed her to say hope you had a safe journey and I have not heard anything since. Some guests just get on with their holiday / stay and you don't hear from them.
If you are concerned I would email them again to say hi and you are just checking everything is as they expected and should they have any questions or problems to contact you. You could also just ask them to confirm they have arrived safely.
If you are still concerned, do you have a cleaner / housekeeper who could knock on the door to check, however its not a good idea to let yourself in as they will have their personal items and you don't want them being upset that someone let themselves in whilst they were out enjoying themselves.
I hope this helps.
Regards
Rachel