Various tools and procedures might want to access the ESP, but it's unclear which, if any, of these might be running on your system:
- A tool or process that installs a new boot loader or that edits a boot loader configuration would need to access the ESP.
- A backup tool -- if you ran one backup tool and are now running another, the first might have left the ESP mounted.
- Miscellaneous low-level system utilities might install tools to the ESP to help with disaster recovery or to help manage the boot process.
- A boot kit (virus) might access the ESP.
Obviously, the last of those is undesirable, but the others are likely to be legitimate. These are just some possibilities that spring to mind; there may be others that haven't occurred to me.
You'll have to ask yourself what tools you have been running that might fall into any of these categories. Nobody here can answer that question without knowing precisely and completely what you've been doing with the computer -- and such a list of activities is likely to be awkward for you to produce and very tedious for others to read.
One more comment: It's conceivable that your computer vendor has installed software that mounts the ESP without your knowledge. If so, you'll have to contact them to find out what it is and how to deal with the situation.