Others have mentioned and I will too- having worked in Home Care and Hospice, it is not unusual at all for the loved one to pass at their own timing when their primary caregiver has left the room. I think partly they relax and are ready to reach for the hand reaching down and partly they want to spare their loved one more distress.
Two stories- a hospice nurse told me that the husband had left the room, the wife looked up at her and said, “I’m ready, you’ll stay with him?”, the nurse said, “Yes I will, I’ll stay with him”. She closed her eyes and passed. Next- a hospice nurse told me our patient had died on Mother’s Day, the patient had really been looking forward to the day, the whole family had planned to gather at her house and enjoy a big family get-together. The nurse told me the meal was over, the “girls” were in the kitchen cleaning up, the guys had taken the nephews and nieces outside to play, Mom was in her comfy living room chair. One daughter came in to see if Mom needed anything and she had passed.
Please temper expectations that you’ll be there at the final moment, your loved one may have something else in mind. Wishing you peace and comfort.