For my mother-in-law who would sometimes get up in the middle of the night to go to the bathroom or once, in the early morning, to wander outside, I bought the Early-Alert Bed Alarm System by Lunderg (available on their website, Amazon, and eBay, among other places). It's a pad you place under the sheet over the mattress, connected wirelessly to a monitor that beeps (two volume levels) when the person intents to get out of bed. There is another one that doesn't alert so fast for people who move a lot in their sleep. There is also one for armchairs. They even sell a package of the bed and armcharir alarm together. You can connect up to three pads to one monitor. It has served us very well, giving us the necessary time to go to her side when she tries to get out of bed in the middle of the night.
On the Lunderg website: https://lunderg.com/bed-alarm/.