>Off the Beaten Path>Is it stealing?
While technically a theft was committed here, incarceration appears unlikely. If caught in the act by the mustard's owner, you might have to pay restitution or disgorge the fruits of the crime.
More likely that a criminal charge in an action sounding in trover. You would not be required to replace the bottle of mustard. The correct measure of damages, in the modern view, is the converted mustard's market value, not its replacement value.
“Everything you need is already inside.” -- Bill Bowerman