It's always a good idea to take a look around your work site before digging. Make sure that what you see is reflected in the Positive Response System codes. If something doesn't look right to you, STOP and check it out. You can never be too safe.
During site inspection, you should also look for depressions in the ground that are not marked. They may be signs of an old facility. Any utility contractor can tell stories of discovering things that are not marked or don't show up on any map. Use your detective skills.
If you're working near a gas pipeline, the following situations could indicate a gas leak. Contact the gas pipeline owner. Digging into this could create a life-threatening condition.