On Fishback Hill near Monmouth today I had what to the eyes seemed to be very much like a juvenile Cassin's Vireo, skulking in the grass and sitting on fences and hedgerows, which I've been told is highly unusual. It's call was virtually identical to that of a juvenile Bell's Vireo on Xeno-Canto, but this may be shared among Vireos. I have attached a medium-quality recording below (including some pishing by me in the middle ;)). The bird had yellow more towards the rear underside than most Cassin's, a lighter eyering and gray hood, and whitish extending up the side of the neck behind the auriculars.
I'll be heading back there tomorrow, and I can give the exact location to anyone who's interested, but it is very likely just an odd young Cassin's Vireo