I found these mines on Lime leaves a few weeks ago but they will still be visible on the yellowing leaves, just not occupied any more. The tiny white tick shape is distinctive, making it easy to identify these as the mines of the little brown and yellow moth Bucculatrix thoracella.

After the ‘tick’ has been formed, the moth larvae graze beneath the leaves, creating small bare areas known as feeding windows. When they moult, the larvae retreat to cocoons – one of the leafmining experts calls them ‘cocoonets’ (shown below), but their final cocoons can be found either in the leaf litter below the tree or on the Lime’s trunk. I’ve yet to find one of those.