The most vulnerable offices are those where the person working is located in the back, with no window to the entrance of the building, and where only one person is working at any given time. Some things can be done, of course, to minimize the impact of being alone in the Leasing Office, but if it's a 1-person set-up, you may always be at some risk.
Start with having a Safety Plan in place: change your set-up so that you can see the entrance. Install an automatic door chime, so every time the door opens, you at least hear it. Some offices will tell the Leasing Person to lock the door if they need to go to the restroom even, simply because if you are alone and leave to use the restroom, someone could walk in and you may not know it. Many Leasing Offices post signage that states No Cash is accepted or kept on the premises. Nowadays, some companies encourage on-line and credit/debit card payments. Some Leasing Offices change their hours of operation, especially for tours, seasonally. Using radios with the Maintenance Team and having a code word to shout if there is a problem works, too.
There should be a written policy regarding proper safety procedures. However, people who work alone should be extra vigilant about their own safety. All personnel should be trained on what to do as well. No one ever thinks they'll need to use 9-1-1 until they need it and get all flustered and just don't remember! It is kind of like having a fire escape plan - write it, post it, and practice it. A lot of times just going to the bank to make a deposit could put you at risk. I have always changed that up and have learned not to go at the same time each day. When walking on site, managers should always be aware. Some properties are actually divided by a busy street. Take care not to walk alone at night without telling someone where you are going on the property and it is a good idea to check in with Maintenance when you get to your destination. I will tell you, I have never had a maintenance team that minded checking on the Office team during the day and especially toward the end of the day before everyone is scheduled to leave.
In that same thought, IF you are the only one in an Office and your Maintenance Techs leave an hour before you close your office for the day, you need to discuss with your Corporate Office whether this is prudent if your property is in a high crime area and you have no Courtesy Officers. Always carry a cell phone (that has charge left on the battery) with you when walking onsite, day or night.
Practicing what you would do in an emergency, is very important and it should be a part of every training protocol for every property.