In future, have in your house rules that you will be entering the premises between 11 am and 3 pm about every 3rd to 7th day to replenish supplies, provide clean linens and towels and to check on the status of light fixtures, appliances and other amenities. State in your house rules that guests must notify you if they prefer privacy during those hours on any given day. Do knock when you arrive and give the guest plenty of time to respond to the door before letting yourself in.
If it is not already in your house rules you can't really go in this time without permission unless you know there is a plumbing, electrical or other emergency. You can enter with 24 hours notice for a valid reason but checking up on them is not a valid reason for a landlord to enter the premises.
You could try going by and knocking on the door. If they are not there, leaving a note asking them to get in touch with you. Seek permission to enter. If they decline or do not respond then you need an emergency or a valid reason with 24 hour notice.