Smell of 'moth-balls'

Observation - Smell of 'moth-balls'. Description: While walking in hornbeam coppice in the Colne Valley (nr Colchester), I was stopped short by the distinct odour of moth-balls in a very local area- my companion did the same quite independently of me. No obvious source was evident. I encountered it again

While walking in hornbeam coppice in the Colne Valley (nr Colchester), I was stopped short by the distinct odour of moth-balls in a very local area- my companion did the same quite independently of me. No obvious source was evident. I encountered it again at the edge of old pasture when walking a couple of days later in Surrey. Both places were 'clean' environments, so surely a natural source....fungal?