Jimmy T – As a frequent visitor to the DC area (and Arlington in particular) the locks that you see outside of apartment blocks are generally not for AirBNB tenants but are for real estate agents who are showing their clients through properties that are for sale or for normal rental within those buildings.
Often properties are listed with many agents. Rather than each agent having a set of keys the keys are locked within a ‘lock box’ and only the code to open the ‘lock box’ needs to be sent to the various agents.
I agree that some may be for AirBNB but generally the majority are for legitimate sellers or rental agents.