What could cause an unknown icon to appear in my iMac's Menu Bar?

It is a third party app, for sure.

It's strange that it shows a version number, but no name. Are you sure it does not say anything else?

Since it does not appear in the things that open at login, it is most likely launched by a Launch Agent, User Launch Agent or Launch Daemon.

You can look at their respective folders (/Library/LaunchAgents; ~/Library/LaunchAgents; /Library/LaunchDaemons)

but there is a good chance it may be unclear what you are looking at. Some of these should be left there, so do not indiscriminately delete anything you are not sure about!

I recommend that you run Etrecheck and post its report here. Then we can look at the report and have a pretty good idea of what this is.

You will NOT lose any of your documents, photos, mail, music, as it is all stored within your own user folder.

You will have to reinstall any third party applications that you use, but that's it.

AND you get rid of a lot of cruft that has accumulated over the last decade and is bogging down your system.

@g_wolfman: while interleaving is a thing, the benefits of having, say, four 4GB sticks of RAM instead of one 16 GB would be marginal, compared to all the rest.

I have no idea if that is an Apple or a third-party icon.

Go to Apple > System Settings > General > Login Items and review the Items that are set to Open at Login.

see > Remove login items to resolve startup problems on your Mac - Apple Support

+1 good call Luis.

@ Adam, I also agree that you should run EtreCheckPro and post the report.

Download and run the free version of EtreCheckPro, from > https://etrecheck.com/en/index.html

Then post your Report, as per > How to use the Add Text Feature When Post… - Apple Community

Anything that appears on this list needs immediate attention.

Unsigned files - There are unsigned software files installed that could be malicious and should be reviewed.

Automatic updates disabled - Automatic updates are disabled. This computer is at risk of malware infection.

Security updates disabled - Security updates are disabled. This computer is at risk of malware infection.

Apple security disabled - Apple security software is disabled. This computer is at risk of malware infection.

Disabling system security is not a good idea...

Also: you have a multitude of system extensions, kernel extensions, agents...

This mac has been upgraded in place for many years, and I am sure this is after transferring stuff from a previous mac. There are items in your setup that were built with the 10.8 SDK, so well over a decade ago.

xquartz, for example, is from 2012... and almost certainly that version does not work in a current OS.

You still have things like RealPlayer, DivX and Silverlight... all of which have been abandoned for several years.

To put it bluntly, your system is a mess of old stuff and new, many that you are surely not aware that you have, or what they were for.

I don't blame you... it is so easy to migrate to a new mac, and upgrade the OS every year.

The problem is cruft like this accumulates over time, and at some point will fail.

It may look like a chore but at this point you should consider:

1) a full backup or three

2) make a system install disk on a usb thumb drive

3) start from it, completely erase the internal, and install a fresh system

4) migrate from the backup, but ONLY the user accounts (*)

5) install the stuff you really use

(*) if in step 4 you were to check all the boxes, you'd just be back to square 1!

Beyond your software and the question of whether you reinstall your OS or spend the time and effort to learn what and how to clean it up yourself...

Your RAM is a problem. You have four RAM slots in the computer holding 4GB, empty, 4 GB and 16 GB. Your memory performance is going to be terrible. Because of something called interleaving, you would be better off having 4GB RAM modules in each of the four slots and only 16GB total than having 24GB in that configuration.

It is maybe not the worst performance if slots 1 and 3 interleave (quite poor if it's 1 and 2 and then 3 and 4 that interleave...but given the cost of RAM these days, I'd recommend buying 4 matching 8GB modules and going to 32GB with all slots being fully and effectively used.

All the software issues are undoubtedly the major issue. I'd have to disagree about fixing the RAM having only a marginal benefit though. Marginal if all the other issues are unaddressed, maybe.

If all the other problems are addressed, balancing out the RAM will still have an additional noticeable improvement compared to the current configuration. At the moment memory interleaving can't work and the way that slots 1-4 are allocated into the A and B channels means that the 4GB sticks will often I/O bind the 16GB stick.

I'd have to assume third-party as it doesn't seem to meet Apple's design specs for its menubar items. And Apple doesn't offer menubar items with no apparent functionality in my experience...