... and get back to sniffing sheets ?
Is it an idiom or something or just was only said here?

I understand it as like a sort of "forget about it".

A private detective Strike was asked to check the police investigation, which had been already done:

"...You think I wouldn't have liked to head up the Lula Landry murder investigation?
Well, sometimes the truth is boring a girl with mental health issues, a pressure job, and boyfriend trouble topped herself.
My advice would be, take whatever that arse head brother's already given you, and get back to sniffing sheets."

You can hear it at the end of this:
https://drive.google.com/open?id=1zC5WEap2nQb1uiIai9gvB96_yGJeyI_-

Question

... and get back to sniffing sheets ?
Is it an idiom or something or just was only said here?

I understand it as like a sort of "forget about it".

A private detective Strike was asked to check the police investigation, which had been already done:

"...You think I wouldn't have liked to head up the Lula Landry murder investigation? 
Well, sometimes the truth is boring a girl with mental health issues, a pressure job, and boyfriend trouble topped herself.
My advice would be, take whatever that arse head brother's already given you, and get back to sniffing sheets."

You can hear it at the end of this:
https://drive.google.com/open?id=1zC5WEap2nQb1uiIai9gvB96_yGJeyI_-

Answer

That thing you think it means, Igor...yeah, that's what it means

Answer

I think he means 'get back to the boring adultery cases most private investigators spend their time on.'

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