Job decorum about vacancy?

Feeling kinda stymied at my current job, but one of the assistant managers just gave her two weeks notice. I was let in on this fact by another manager, who encouraged me to apply to that spot. He also said the job vacancy might not be formally posted since the head manager wanted to bring in some old friends from their former job. Would it be bad form to express interest, since I wasn’t technically supposed to know yet? What is the best strategy here? Thanks for any advice!

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