Fort in Wales discovered due to coins and drought

[Epictetus]

From the BBC:

The Roman fort complex discovery near Brecon was a "rare discovery for Wales" and was made following a tip from Dr Jeffrey Davies, who he has been working with on another project - the Abermagwr Roman villa excavations near Aberystwyth.

"Jeffrey Davies noticed an anomaly in Roman coin finds near Brecon, reported under the Portable Antiquities Scheme (PAS)," explained the aerial archaeologist.

"He had a hunch that the coins, of the Emperor Claudius, could indicate a lost early Roman fort, and passed a grid reference to me the day before a flight into central Wales.

"I couldn't believe my eyes when the pilot and I approached the location and saw fading crop marks of a major Roman fort complex, lost beneath fields and a road for nearly 2,000 years."

Between Caerwent and Chepstow, the aerial survey pinpointed only the second Roman overnight marching camp in Monmouthshire which Dr Driver said appears to show a small expeditionary force on manoeuvres, perhaps in the years around 50 AD.

"Because the campaigns against the tenacious Silures were documented by Roman historians, we expect more camps in south east Wales than we currently know about," he added.

[img width=400]http://news.bbcimg.co.uk/media/images/69213000/jpg/_69213128_aerialpic_1.jpg[/img]

Interesting story, but I'm confused by this 'coin anomaly' thing. How could coins indicate a fort nearby?