Your tenants have a right to peaceful enjoyment of the property they rent and you have a responsibility to ensure that right is observed. This means you cannot enter the property without their permission and you cannot allow others to access the property.
If you suspect that your tenant has abandoned the property, you should be very careful. You cannot regain vacant possession of the property without a court order. You may wish to seek legal advice in these circumstances to protect yourself against claims of illegal eviction.