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Can you elaborate on what you meant by trackable when they're "continously in close proximity to you"?

Snooping, or hijacking, the radio pathway between you and your network operator.

https://en.wikipedia.org/wiki/IMSI-catcher

If they can listen to the radio traffic between your phone and the cell tower, that is.

I think this is only true for older mobile networks. In 4G and 5G, I don't think the IMEI is part of any unencrypted radio message anymore.

Even the IMSI is only used when absolutely necessary, i.e. for the initial attachment procedure when cold starting a device or entering a new routing area; after that, it's replaced by an alias called TMSI to make tracking phone users a bit harder.

New Android versions will supposedly have a switch in their settings to show a warning every time the IMEI or IMSI is transmitted in plantext [1].

[1] https://cs.android.com/android/platform/superproject/main/+/...

Ah ok I had a police officer force me to unlock my phone and show him my IMEI once so that answers why.