Hmm. Do you have anyone with kids that goes over your mother's house? Maybe a kid went in to your room while they were visiting and saw them on top of the boxes and took them to play with.
Only because it seems very odd that your mother would take them, not just move them to somewhere you should have found them, but if a little kid saw them sitting on top out of the boxes, their mother might have just scooped them up with the rest of the kid's toys at the end of the night an taken them home not realizing they didn't belong to them.
Maybe ask who with kids has been over the house? And then follow up with the parents to see if they have them?