I've had a Fitbit for several years - and I admit I'm addicted to checking the number of steps I take each day. So far I've resisted the urge to get the apple watch - but they look pretty cool. There a few people these days walking around without some type of technology - whether it be a phone, or a device like a Fitbit. Since those devices are almost always on you, it will only became a matter of time before the data they contain starts being used in court.
Cell phone location information has been used for several years, and the courts are still trying to determine how to address the privacy interests - or whether you have any interest in the data your phone transmits. In case you don't know, you phone is constantly emitting signals, and connecting to towers. Some information is more specific than others, but in general, your phone can be narrowed down to the area covered by a particular cell tower. Your movements can be tracked by looking at which cell towers you were utilizing.
Wearable devices, and device like Alexa and Google Home offer a different kind of information. The data in those devices may support the story you give police, or cast doubt on it.
The use of this technology is front and center in a murder case in Virginia. George Burch is on trial for murder of young woman. Part of the evidence against him consists of data from his cell phone - in this case a Samsung. He is having a hard time explaining whey he just happened to be in several places at the same time the victim was, including the location where she was found. On the other hand, the victim's ex-boyfriend was able to use his Fitbit to prove his story. He was initially a suspect, until the police finally accepted the story he provided - which was backed up by a data on his Fitbit. Something that was noticed when someone was looking at the video of his interview, and noticed the device on his writ.
I believe we are going to see more of this type of evidence. So before you commit yourself to being somewhere, just remember you may be wearing the evidence that contradicts you.