Like every rental property owner in St. Charles, you want to be viewed as a good landlord. That includes being respectful of your tenants' rights. At the same time, your tenants are occupying properties that you own (and ultimately, you have to manage and repair once they are gone). Thus, your desire to be able to enter your rental units to perform routine inspections is understandable. There are also may be times when you need immediate access to a certain area of a unit (such as a shared utility area), or where you suspect tenants may be engaged in illegal activity. The question then becomes with rights of entry do you have to occupied rental units?
Most states have established laws that dictate when a landlord can enter his or her rental properties. Unfortunately (according to the website Landlord.com), Missouri has not. Thus, it offers recommendations that are best off of best judgment and simple decorum. For example, if you need to enter one of your units (with or without consent of the tenant), inform the tenant at least 24 hours ahead of time of your intention to do so. Also, do not try to enter at odd hours, and always afford the tenant the courtesy of knocking before unlocking the door.
Try and limit unconsented entries to emergent situations, such as addressing emergency repairs. If you do not want the tenant to be there, inform him or her of your wishes. None of these suggestions are meant to take away from your rights as a property owner, nor should they be viewed as legal advice, but rather suggestions on helping you to avoid confrontations that could quickly turn contentious.