I wonder if it coincides with another animal or insect coming out at that time of night, or is she associating something that happened when you were cooking with the smell in the air.
In either case , given her age, you have to decide how important it is to change. Aside from her wanting to leave the area, is it disrupting her life necessities? If it is not then I would not do too much at this time to try to make her change because overall you could make things worse.
However you can try the steps of desensitizing by making that place a good place to be in at that specific time of night.
You can try to have her make it through the area before she has the time to stop and smell, this keeps her mind moving forward. I would put her on leash and give a command such as heel, and reward that heel with a special treat that she will only get at this time.
You can then try laying some of that treat on the ground through the area so she concentrates on that smell and follows that path with her nose to the ground looking for the next treat. It is possible that once she does this on her own a few times, she will break the cycle.
The other possibility that I wonder about given her age, is if she is starting to go through canine cognitive dysfunction (CCD) or sundowners which will have the dog show strange behaviors.
CCD is not usually noticed too much by the owner when it starts, but usually where there is a drastic change in behavior or when the dog seems to forget its potty routine and begins to go inside the hoe is when the owner notices it.
I would try the above suggestions and then if there is no change with this or you notice other odd behavior talk to your vet about CCD.