。
The light was left on, you know.
Literal
Light [subject-が] left-on-[state-っぱなし] [copula-でした] [assertive-よ].
つけっぱなしでした — noun-style predicate 'was [in the state of being] left on.' The ~っぱなし pattern has grown into a productive noun marker for 'a state of having been left X.' The sentence-final よ offers the information as a gentle reminder.