The Fitbit watch Nicola Bulley was wearing still gave a heart rate output for eight days after she drowned, an inquest into her death has heard.
The 45-year-old went missing after dropping her daughters at school and going on a dog walk on January 27 along the River Wyre in St Michael's, Lancashire. Her phone was still connected to a Teams phone call with work and her dog Willow was found nearby.
A huge search operation was launched and the mum of two's body was found in the river about a mile from the bench on February 19.
The coroner's court was told today that she drowned after going into a cold water river and there is no evidence anyone else was involved in her death, the Mirror reports.
Nicola's Fitbit was still attached to her wrist when she was found and is believed to have lost power on February 4 as no further data had been obtained beyond that point and when it was placed on charge it displayed that date.
On January 27, between 8am and 9:30am, the Fitbit recorded 4,548 steps. After 09.30, no further steps were recorded by the device.
DC Keith Greenhalgh told the first day of the inquest at Preston's County Hall: "A possible explanation for Nicola's Fitbit continuing to provide [heart rate] output sporadically until 4 February could be a result of the movement of water passing between the device and Nicola's wrist.
"Testing on intimate objects provided similar results when there was a movement in the water."
DC Greenhalgh said he believes Nicola fell into the water between 9.18am and 9.30am. He told the inquest analysis of her iPhone and Fitbit data suggests she "very possibly" entered the water at 9.22am on January 27.
Drowning experts told the hearing entering into cold water can cause a victim to gasp and inhale water and drown within seconds.
Professor Michael Tipton, a world expert in drowning, said there would have been a "fairly rapid incapacitation" after Nicola went into the river, which could have been as cold as 3C. It may have been just seconds before she began to lose consciousness, the hearing was told.
Prof Tipton said he agreed with Home Office pathologist Alison Armour, who conducted a post-mortem examination and concluded Nicola drowned.
He said Ms Armour's finding of water in the stomach and lungs is consistent with a "gasp response" under water, with liquid entering the body's organs.
It could have been an estimated "20 to 30 seconds" before Nicola's lost consciousness, the inquest was told. Two breaths of water would have been a "lethal dose" for someone of her size, Prof Tipton said.
Cold water expert Dr Patrick Morgan said: "On the occasion that the individual has taken that initial gasp on the surface of the water and then gone below, the duration would be 10 seconds that you could hold your breath and very likely one or two seconds at best."
Pc Matthew Thackray, a police underwater diver, said the area where Nicola is believed to have gone into the water has a steep slope.
He added: "The river was 4C, so almost freezing, and if she fell in, the muscles would probably seize, making it difficult to swim properly."
Nurse Helen O'Neill said: "I heard a scream, it's not an alarming noise, it was just over in a couple of seconds.
"There were no other sounds for me to be concerned about."
Veronica Claesen said: "I was just about to get into the car and I heard a scream. A very short scream and my immediate thought was, 'Somebody is having a bit of fun at the back of the graveyard'."
Nicola, a mortgage adviser originally from near Chelmsford and living inInskip, was immediately deemed a "high risk" missing person when she disappeared.
Nicola's partner Paul Ansell will give evidence on Tuesday at Preston's County Hall.
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