Brat,
I am sorry for youre dillema. The day you picked up youre meds you signed for them , right?. The state I live in makes you sign for every script you pick up at that time.So the Pharm should have a record of the RX's that you signed for. Also they give you receipts for each med. So please for my sake do not take this to heart.
If all of the things I mentioned above are true , I would do one of two things to relieve youre stress.
1) If youre Grandma has a legal guardian , maybe she could go to the pharm and ask for the records of meds received in the past two months. They can usually print that out in seconds.
2) File a police report if it is a controlled substance. I have a wife , 3 children all with cronic conditions. Many mistakes have been made over the years. In fact we just changed pharms because I was accused of picking up my meds when in fact they were given to an elderly gentleman who thankfully returned them the next afternoon. Up until that point the owner was blaming me , yet his high school help had pulled the bag containing the meds, while I was talking with the pharmacist over at the other end of the counter. It was a controlled med and the owner threatened calling the police that morning. I asked him to wait and sure enouugh the bag showed up. No apology from the owner , now he lost our business. Which was quite good. But my point is if this has gone one for a couple of weeks, and no resolution. Call the police. Somebody took the meds. I've seen and heard quite a few stories that go on at pharms.

If this store has a delivery service, it is possible that a delivery person picked the bag up and took it. A lot of pharms put the bags in plain view behind the counter. Now I have lousy vision, but the pharm I used before , you could easily read the name of the person and the drug right from the counter. So someone could request it that way. Especially if they don't know Grandma face to face. So if I was you , it sounds like it's eating at you and could cause a family rift. You don't want that. Settle it once and for all.

Sorry for the long post. I feel youre pain having been accused myself of stealing meds only to find out they were given to the wrong person. Best of luck.
Best Regards,
Hoop