This may not be much of a spoiler, but halfway through the episode, Felicity’s and Oliver’s house has the security system that said “Armed” and then Oliver walked in. Not a very safe or secure system.
Maybe it can recognize Oliver via bio scan or something?
A couple episodes later, the security system is referenced, but it still doesn’t explain how the alarm can be set to “Armed” without the loud siren that usually blares.
I believe the security system is programed a little like Archer which can track people via DNA so maybe when Oliver approached his DNA was scanned
I just watched an episode where Felicity walks in saying it was disarmed, then armed. Oh well, I prefer the security systems that have a siren, not this new stuff that I don’t understand.