Hi, I own a cleaning company and this would be a big no-no to me. Many clients report of past cleaners doing this, or other people we know with other cleaners are doing/have done this and it completely breaches trust.
However, personally I'd agree with a first approach of sending a message as some others have suggested saying you got a notification she left early, hope everything was ok. I'd see what she said in response and if it was some kind of emergency, I'd then say ok but can you please send me a message if that happens in the future (particularly if significant time has passed, then I'd expect someone to have time to notify there was an emergency. Perhaps not right away if it's something urgent, but definitely to get in touch ASAP before you have the opportunity to pay her)
If she doesn't state emergency and you figure it's from doing the job quicker, she still should be staying the full agreed duration, in which case I'd suggest what other tasks she could do to make up the time if this happens in future, or to use her initiative and do extras she saw fit up until the time. Either that or if she is unwilling or turns out she'd prefer to revise the time allocated, then suggest cutting the time & therefore cost down.
If she's not let you down knowingly before, I'd do that and by communicating so, you make it clear this isn't what you agreed to. After the response I'd ask what I owe her for the 1 hour 20 mins and go from there.
Definitely best to have clear communication straight away, you're paying for a service at the end of the day, charged by the hour.