I remember users losing the mini viewer a few years ago. I certainly lost mine when making display changes:
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Using Windows virtual desktop/display feature that is built into Windows 10. Note that this is not the same as the Microsoft Azure Virtual Desktop
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When changing the scale of the screen resolution.
If neither of these features are being used then I’d also suggest reinstalling GlassWire.
@Ken_GlassWire, there was also a GlassWire/video driver problem that was fixed. I’d be surprised if it is the problem but maybe there is a similar video driver issue: Mini viewer is off-screen somewhere