、、。
I'm really sorry, but it seems I've gone and left your scarf somewhere.
Literal
Truly bad-[explanation-んだ]-but, your scarf, somewhere [at-に] put-came-[completion-てしまった]-[seems-らしい]-[explanation-んだ]-[insistence-よ].
A richly layered colloquial apology. 悪いんだけど is a standard softener for bad news ('sorry, but...'). 置いてきてしまった chains three auxiliaries: 置く ('put down') + ~てくる ('do and come back to here,' meaning the action took place elsewhere and the doer returned) + ~てしまう ('do to regrettable completion'). らしい adds inference/hearsay ('it seems'), distancing the speaker from full responsibility. Final ~んだよ asserts this as explanation. 襟巻き ('scarf') is an old-fashioned native word; younger speakers say マフラー.