abandoning a store unsecured is not a good plan. I would not recommend working another shirt without having a key so that you can lock the door if needed. Then, if no one shows up, you can call the manager. Leave a message that you are staying 30 minutes and then will lock the door, set the alarm (if possible) and leave.
You should be able to secure the doors this issue aside. If you are working there alone, for your safety, you need to be able to lock the door.
Can you be sued in civil court. Yes, people sue all the time. Can criminal charges be filed? Yes, if they feel you walked out just so others could come in and steal. This would not be a good idea. The best thing to do is find another job. Walking out on your shift leaving the store unattended could mean legal trouble. Even if you prevail on the civil claim against you and the criminal charges are dismissed.... you had to hire an attorney, post bond on the criminal case, go to court, etc. You should find other employment.
The information provided is not advice but a legal perspective and you should schedule a consultation with the lawyer of your choice.
As others have stated abandoning a store unsecured is not a good plan. You risk both civil and criminal liability. Make sure you do not work another shift without having a key so that if your relief doesn't show up for their shift you can lock the door and leave. In this manner you can properly secure the premises and not merely abandon it. Once you lock up the store call your manager and notify them that you are leaving and resigning from your job. If the premises is alarmed set it.