I have worked wit the older JCMs, but they are no longer operating in NZ, so I haven't played with newer models.
It sounds like you have a staff member who is in the middle of a sale. Usually you would log on as each staff member, and try to finish a sale with CASH. If one of them has a sale in progress, the drawer will open. Some machines have a clerk read option that will tell you the status of staff, so you would need to find it in the reports section.
If you have an electronic version of the manual, and could send it, I could help further.