This specific individual did not show very pronounced streaking on the chest, being rather similar to a Grasshopper Warbler. Note the sharp contrast between the narrow pale borders and the black centred tertials. This field mark is good for identification compared to Grasshopper Warbler Locustella naevia which has more diffuse borders.
The picture below show the habitat where it stayed. And after having been found in the field it was caught and ringed.