Blank Donation Slip

Processing a batch of donations, one envelope was opened it had a small amount of cash, no name, no membership number. The envelope was also blank.

What have you done?

We would make an announcement in Sacrament meeting next week and see if anyone claimed it, using almost the exact words you used in your post.

We would do as TinMan says with one exception. We would have processed it using the donor name as "Unknown" so that there is no money just sitting around. Once we found out who the proper donor is and what category the donation should be place in, we would then go into the closed batch and make a correction.

From the wiki Donations

Q: What do I do if there is no Tithing and Other Offerings slip in the envelope?
A: Prepare a Tithing and Other Offerings slip for the amount of money in the envelope, and print the word "Unknown" in the name block of the slip. Enter the entire amount in "Tithing" on the slip and in MLS.

If someone steps forward later to claim the donation, you can change it.