It can be found in the Anchorage Memorial, in the utility closet of the Memorial service entrance next to the Anchorage stash key. The actual stash is in a refrigerator below the kitchen/recreation area, where there are a few dead and 'empty' Mirelurks on operating tables and chain hooks hanging from the ceiling. Approach the right hand refrigerator of the two standing side by side, even if it is 'empty', and open it. If you're holding the Anchorage stash key which you got from the Utility closet with the broken door, you can open a hidden compartment for 240 caps and a Mirelurk cake recipe.
The stash is hidden downstairs in one of the freezers. Rocky set it up. You know his style. Don't forget the key -- you'll need it to unlock the compartment.