Chambers get lost under decking, block paving, patios, extensions and lawns. The sonde method finds them to within a few centimetres and also gives the depth, so lifting one slab is usually enough.
Where a chamber has been permanently built over, a recessed or double-sealed cover can often be installed at the new surface level to restore access.
Knowing where your chambers are before you pave, deck or extend saves a great deal of expense later.