My son comes home this past Monday from school and says someone was stealing his sandwich from his lunch box at PE. Did it Friday and Monday.
Asked if he thought he knew who it was. He said this one kid waits to be the last one out of the locker room and then comes out late. He said he was pretty sure it was him.
I first asked if this kid was in need or was having a rough time in life. He said no. He probably ought to skip a few meals. I said alright then let's do this.
I told him when he was in the locker room to dress out sitting right beside this kid and say absolutely nothing. As close as he can get and bump a few elbows accidental. Get up and put all your stuff up like normal and before walking out, stop in front of him, look him stone face in the eyes for 5 seconds then walk out.
He has had his sandwich in his lunchbox every day since.
No fighting. Just intimidation and silent warning.
At first he said it sounded silly. I told him to just try it. If it doesn't work he will know someone else is stealing it.
When it worked, my son kind of giggled. I told him there are other ways to "get your point across". I call him Intimidator now.

I did call the principal and advised him what was going on. Told him I didn't want to be "that" parent but I was concerned if there had been money or new shoes or something valuable in it. Principal said he agreed with me 100% and was gonna check video footage and let PE teachers know what was going on also.