However, it is possible that they were shooting live rounds "at nobody".

Sheet rock wouldn't stop the bullets, so the troops would have no idea of what would/could be hit, therefore, no target, no firing.

Concrete would cause richocets. Again no target, no firing.

Only time we were told to use covering fire was on open terrain, outside where only enemy would be located.

Anyone here with recent "house-to-house" combat training?