I may have downloaded plugins that I don’t remember.

Well, of course there is any easy way to show yourself (and others) your plugins. Looking at your Plugins menu is one way, example:

And going to the ? menu and choosing Debug Info is another way:

It will give you something that looks like this:

I don’t want a session prefix in the title bar

You mention a session prefix. Notepad++ doesn’t do that natively. Are you using the Session Manager plugin? Because that plugin has a session-prefix option (Show in title bar). If you are using that plugin, then I doubt that the plugin obeys Notepad++'s -nosession command line option. Unfortunately, the author of that plugin has abandoned it, and other than someone bumping from 1.4.2 to 1.4.3 to support 64bit Notepad++, no one has adopted the plugin, so there’s little chance of getting a new feature added anytime soon.

addendum: the Session Manager documentation is still hosted here, if you want to learn more about the settings.

I’m using the -nosession flag constantly and it works for me.

If I understand correctly, then setting prefereces->multi-instance to default and uncheck preferences->Backup->remember current session for next launch should do what you want.