Difficult to comment specifically as I'm not from Chiltern ASU, but we often do searches of - [mostly] rural areas for, say, missing persons. This would involve a widespread search without ground patrols being called unless we found something of note. Could be all sorts of reasons though......
Police operating levels aren't limited to Rule 5 as CAP 612 allows considerably lower where justified.
Unlikely to be a Bo105 as Chiltern use an EC135 with [I think] an AS355 as backup.