You also have said you are always first in and first out, but also have implied you don't work all the days the store is open and are not the owner etc.
If it was possible for them to have sold, it could also be possible that others removed the dead fish without properly documenting and without letting you know.
Do you have a need to continue to get minnows? In the past couple weeks when they have disappeared has anyone asked for some? Maybe a minnow break would not harm the store.
I get the feeling you're not reading entire posts or you're just skimming through them and not fully comprehending. I'm not ALWAYS the first in and first out at the store. I've been talking about one 12 hour period of time, on one day of the week. Very specific time frame. Here's how it works....
1. Fish shipment arrives at some point in the afternoon during my evening shift.
2. I put fish in their tanks after floating the bags for approx 5-10 min.
3. Store closes at 9pm all other staff leaves.
4. As part of the end of my shift I check all animals in store are secure, including fish. All minnows are present and comfortable in tank.
5. I leave store and lock up behind me.
6. I go home and go to bed cus I have to be back at the store at 8:30am the next day.
7. 8:30 the next day, I unlock doors, turn on all lights and check animals. Minnows are gone.
So to summarize in case you didn't get it. The minnows disappeared during a single 12 hour period, overnight, when the store was locked and alarmed, and no one else was around.
And yes I have to continue to get minnows. They are an excellent beginner fish, hardy, and no I don't get people that ask specifically for them. However most people don't shop for specific fish. They just go in looking and want something "pretty" It's my job to make sure they have a good selection and that they choose the right fish for their tank and abilities.