No, I just posted this for the hell of it.

..and what software are you using to remove the devices ?
So far this has been about bluetooth manager software on Lunux.

I just noticed that jan128 is using paired and detected as equivalent terms. They are not.

Previously paired devices can be removed with the existing management software.

The computer displays devices it detects because they are “detectable”. That’s how one can choose to pair/connect to them. I guess I don’t understand why one would want to suppress or remove those, but yes, the only way I can think of to have those not show up is to turn off the Bluetooth device so the ‘puter can no longer “see” it.

Note that removing/unpairing a device that is in range will not take it off the list if it still “detectable”

SOLVED
int hci_inquiry(int dev_id, int len, int max_rsp, const uint8_t *lap,
inquiry_info **ii, long flags);

hci_inquiry is one of the few functions that requires the use of a resource number instead of an open socket, so we use the dev_id returned by hci_get_route. The inquiry lasts for at most 1.28 * len seconds, and at most max_rsp devices will be returned in the output parameter ii, which must be large enough to accommodate max_rsp results. We suggest using a max_rsp of 255 for a standard 10.24 second inquiry.

If flags is set to IREQ_CACHE_FLUSH, then the cache of previously detected devices is flushed before performing the current inquiry. Otherwise, if flags is set to 0, then the results of previous inquiries may be returned, even if the devices aren't in range anymore.

I use the standard bluetooth-manager, and right-clicking on a device brings up a menu including "forget the device". Select that and the device is gone from sight. But a search will return it if it is detectable.

More I follow this , more I believe it is Linux / bluetooth-manager version / flavor dependent.
Right now I have phony BT device / paired and "right click" on it does nothing.

However, since I am using C code , it really does not matter to me how bluetooth-manager interacts with OS.
I was using it as a demo to show how it keeps detecting non-existed devices.
I am finding other anomalies (timing) with Qt Bluetooth module and gradually abandoning it whenever feasible.

This entire encounter has been a peculiar challenge and so far NOBODY came up with (correct) description how and where does Linux keeps the "discovered bluetooth devices " list.

But "hci-inquiry" delete such list and that is what counts. ( Time to look at "hci-inquiry" source code ...)

I have posted a bug to Qt -twice - , but so far no takers.