1. A security system comprising; a first information collecting module configured to cyclically collect, as target-object information, information indicative of environments of a target object and a monitored area in which the target object exists; an alarm module configured to: determine whether there is an abnormal situation associated with at least one of the target object and the monitored area based on the target-object information collected by the first information collecting module, and set off an alarm upon determination that there is an abnormal situation associated with at least one of the target object and the monitored area; a second information collecting module confirmed to cyclically collect, as circumstance information, information indicative of a situation of an area surrounding the monitored area; and an abnormal situation determining module configured to: calculate, based on the circumstance information collected by the second formation collecting module, a probability that an abnormal situation associated with at least one of the target object and the monitored area will occur; and determine whether the probability that an abnormal situation associated with at least one of the target object and the monitored area will occur is equal to or higher than a threshold level, wherein the second information collecting module is installed in a mobile object, and comprises: a first circumstance information obtaining module configured to cyclically obtain, as mobile-object circumstance information, information indicative of at least part of circumstances around the mobile object; and a second circumstance information obtaining module configured to cyclically obtain, as behavior information, information indicative of a behavior of the mobile object, the second information collecting module being configured to: determine whether the mobile object is located within the surrounding area; and cyclically collect, as the circumstance information, at least one of: a piece of the mobile-object circumstance information obtained by the first circumstance information obtaining module; and a piece of the behavior information obtained by the second circumstance information collecting module until it is determined that the mobile object is located within the surrounding area.