Yes, they will enter a home during a welfare check, preferably getting someone to let them in, but they'll go in through a window (the person may be inside dying from a drug overdose, for eg).I'm just curious about the timeline. She misses her flight. Her parents call LE to do a Welfare check. What time did they show up at her apt to do the check? Do police go in to locked homes for welfare checks? If they see no signs of forced entry or anything suspect, do they still enter the home? Did the LEO ask a property manager to allow him access? Did the property manager check activity for her apt locking mechanism and tell LE that one of their own maintenance men had entered her apt earlier? What was LE thinking during the check? Miya was playing a game of hide and seek? Her apt is a mess and her room is barricaded. None of that seemed suspicious? Lots of questions here.
IMO, they will look for evidence of whether the person left voluntarily. That will often be a missing phone/wallet/purse/etc. They would look for anything obviously suspicious for eg a suicide note, a trail of blood. They don't know the person, they don't know what happened, they can't jump to conclusions based on a bureau being moved or a window being wonky. She could have been in a car accident, abducted by an uber on her way to the airport, suicidal (2nd cause of death for young people after accidental). Or she could have had a secret life the family didn't know about.
We don't know when/how the evidence of the master key came out, but it doesn't appear to have been right away. The media only report that Caballero killed himself after he found out that there was evidence about the master key. From that video of the encounter between him and the family, there was as yet no evidence about the master key, and that must have been after the initial check on her apartment, since the family had come down to Orlando themselves.