For some reason I've always got a chalk squiggle on my bag in BOM and used to wipe it off like most other people. And I never have anything valuable in my checked bag - but often do have a bottle of wine. Last time I couldn't be bothered to find the wet wipes somewhere deep in my backpack and pointed the chalk mark out to the guy at the scanner while exiting and said I supposed I'd have to have the bag scanned. But he waved me through.