Okay I am fully prepared to get flamed her but I can't think of any other option. Dss mother is erratic and bizarre. I have caught following me around before. She has been caught looking at my and my dc Facebook and we have not only had to block her but her son as well because she will use his account.
I would not put it past her picking up the key from her sons pocket and coming round when no one is in.
Dss goes to the same school as my son so they make same journey home.