When are Drexel Alerts sent out?
Ideally the dates and times the alerts were sent would reflect the time that the event occurred, but unfortunately they do not. I have no special insight into Drexel's alert system, but the one event that I witnessed in real time took several hours to come through as a Drexel Alert. This was a major event, so it is likely that this timing was an anomaly.
Map of alerts by time of day (click for zoom-able map)
On this map, red alerts occurred at night and green alerts occurred during the day. Yellow alerts are during transition times (may be light or dark outside depending on the time of year).
Looking at the map of the times that the various alerts occurred, there doesn't appear to be any pattern.
Types of activity addressed by alerts
The most common type of alert that Drexel Alerts are sent out are regarding robberies.
Taking all these things into consideration, and assuming that robberies described as "strong arm" don't involve a weapon, you are only slightly more likely to be robbed by someone who is unarmed.
As for where different robberies are committed, that can be seen on the map below.
Map of alerts by type of activity (click for zoom-able map)
Police Activity (blue), Robbery (yellow), Assault (red), Theft (green), Gunshot (pink)
Interestingly there does seem to be a correlation between activity and location. Most of the robberies are located inside Powelton Village between Powelton Ave and Spring Garden. This is where a large number of Drexel students live and isn't as open as the heart of campus. The alerts about police activity only come from right on campus near the academic buildings, although there is definitely a lot of police activity that there are no alerts about.