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 become 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 privacy interest in the data your phone transmits. In case you don't know, your 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 others like Alexa and Google Home offer different kinds 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 the murder of a 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 why 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 data on his Fitbit, which was noticed when someone was looking at the video of his interview and noticed the device on his wrist.
I believe we are going to see more of this type of evidence. So before you claim you were somewhere, just remember you may be wearing the evidence that contradicts you.