Legal Question in Real Estate Law in Missouri

Your posting is a bit confusing. if you mean can a landlord enter an abandoned residence, the answer is almost always "Yes." Landlords often have keys (or require it in the lease agreement), so it is not really "breaking in." If you mean can they enter when the renter has gone for the day, change the locks and remove the renter's belongings, because the renter is behind in rent, the answer is almost always "No." A landlord that does that, without obtaining an eviction Judgment from the Court, can wind up having to pay the renter much more than the back rent amount.

If the renter has left the premises and left their belongings, few if any courts will enter a Judgment against the landlord for entering (by any reasonable means) to assess determine if the renter has truly moved out, and assess the condition of the premises.

If you are the renter in this story, get your stuff out as soon as you can, and contact an attorney if they are gone or damaged. If you are the landlord; go in, make sure the utilities and HVAC, Plumbing, and lighting systems are functional, and assess the cleanliness and condition of the rental space. Take pictures or video, and try not to remove anything until you get an eviction Judgment.

You were smart to get general information based upon the limited information you could provide here. It might be wise to get legal advice specific to all of the relevant facts of your particular situation.

Good luck