Cameras are so cheap nowadays too, or some types are anyhow and they solve so many crimes, it should be a no brainer to install them. In another case I follow on here, a neighbor in an apartment complex had a camera installed in her "area" outside. I believe the renters installed it themselves. Any hope maybe someone there installed one that may have caught something or have a ring doorbell they installed? I do not think it would likely occur to most renters they could install such a thing and maybe in most complexes they can't.
I am going to assume the locks are deadbolts with regular keys? If so, maybe the handymen can't lock up because they don't have key access? I lived somewhere once pretty big but not as big as what you are talking about and while people came quite often to replace carpets, etc., I am about positive management let them into the apartment and they had no access to the key. Apartments could not be locked upon departure without a key because the locks were keyed deadbolts.
On another recent case on here where a woman is missing, I read reviews on the complex she was last seen at. There were some really good review and a lot of bad ones as well. One renter had hardly moved in and came home after work and found another woman's toiletries all over her bathroom vanity. Not long after that, some of her things were missing. There was a third incident, I forget, but I think she found that maintenance or someone was in there without notice or permission. She moved out and in pretty short order.
Just food for thought. I did read somewhere Carolyn had been there about two years so she was not new to the complex but workers can certainly come and go as can other renters.
Jmo.