These are college violations. Entrance doors to the residence halls should never be propped open. The locks are designed to allow only residence of the hall to have access. Propping doors open will allow non-residences to enter, jeopardizing yours and everyone else's safety. All visitors, student and non-student, are required at all times to be escorted while in a residence hall by the person they are visiting.
All visitors are to be escorted out of the residence halls when leaving.
All crimes need to be reported immediately to your RA, RD or Security. Do not leave notes on your door that you are not there. That is an invitation for some unwanted person to enter your room. Keep your shades drawn when changing clothes or retiring for the night. Purchase locking devices if you have your own computer and printer in your room.