Had same challenge for a customer.
Ended up with a data action in the in-queue periodically checking the queue observation api to collect total onQueue and OffQueue (Any state) counts into an array output and checked in flow for Count() on the array being 0, meaning no one logged in (a member regardless of off queue reason)
Not exactly what you refer to, an alert, but this allowed the call to be moved to another queue after a period in the low volume queue.
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Simon Brown
Maintel Europe Limited
Senior Applications Consultant
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