There are many topics on the forums regarding occupancy sensing, including the recent one on Smart Chairs that you might find interesting.
For a bed, the easiest one will probably be what hospitals typically use: a pressure mat under the mattress. That way it’s not dependent on you carrying a device. If the dog gets on that part of the bed during the day, you can define the time it will trigger or block the area. Of if the dog is small, choose a mat with a higher weight threshhold.
This and other options are discussed in the smart chair topic: