Mobile, or drive through, security guards can serve a useful purpose even though they do not remain onsite and normally make rounds in a car or van. However it is important to be aware of the limitations of a mobile security patrol before deciding if it is sufficient coverage for your needs.
A mobile service is often less expensive than a dedicated onsite security guard because less time is spent on the property. As long as the vehicle is highly visible, a mobile security detail can be an ideal deterrent for commercial areas during out of hours, or even on private property which the public should not be able to access at any time.
There is generally a lot more to a mobile patrol than literally driving around the area. Normally the patrol will observe and, if possible, test, all the doors, gates, windows etc to make sure they are secure, and report problems to the property owner or (where appropriate) the police. A visible mobile patrol also discourages trespassing and vandalism. Some security services can respond to alarm signals but this is generally more specialised. Properties which are visited by mobile patrols will normally have some signs up which announce that the area is patrolled by a particular security service.
It is important to note that mobile patrols are not necessarily good at catching criminals in the act. Mobile security patrols are designed to be highly visible so really are incapable of approaching a perpetrator unnoticed. The best patrol vehicles will be high profile and with a distinctive appearance, well-marked and in many cases equipped with light bars on the root and high-intensity spotlights for sweeping the area. Provided the vehicle follows its designated patrol route, this makes for a very efficient deterrent for would-be intruders.
Mobile patrols are usually less effective on commercial properties during public opening hours, because random and infrequent patrols does not allow the security guards to observer loiterers and successfully tell them apart from normal customers or residents. This means suspicious groups are unlikely to be identified, let alone confronted, so while a mobile drive-through patrol is better than no security at all, do not be tempted into thinking that all is well and entirely secure just because a van drives by the property every half hour and checks that the gates are still locked.