A searcher is a tool, used by the OS, to locate trig blocks. The term appears to have originated in the Principal Triangulation era:
"Station marks of the old Principal Triangulation were usually buried, and only a few of the old primary and secondary stations were provided with some form of mark above ground level which normally consisted of a cairn, and then only when the station was on the top of a high mountain. Subsequent use of the station normally required the use of a special tool, known as a 'searcher', to probe for the buried tile or other mark before attempting to expose it by digging. Owing to rapid change in topographic detail since the Principal Triangulation, and to the impossibility in any case of providing adequate descriptions on bare rounded hills, this operation of searching was frequently protracted and often entirely unsuccessful." 
The use of a searcher by the OS continued. For example, the station inspectors report, dated July 1982, for the Buried Block at Church Lane states: 'Block located with searcher but not opened up'.
The exact appearance of a searcher, and its method of use, are yet to be established. However, it is thought likely to resemble the implement used by modern-day trig baggers for the same purpose - a long-bladed screwdriver which is used to repeatedly probe the target region until an obstruction with a suitably large area is detected.
- History of the Retriangulation of Great Britain, Section 2.060, Loss of station marks of the principal triangulation