Air carrier pilot reported a possible runway incursion while taxiing on Runway 6 with instructions to hold short of Runway 12L. Pilot recommends a 'Hot Spot' flag/designation for this location on the airport diagram to warn of potential for incursion.

Narrative

STL Ground Metering/Ground initially cleared us to taxi to Runway 12L via [taxiway] D; hold short Runway 06. Approaching Runway 06; Ground cleared us onto Runway 06; cross Runway 12R; hold short Runway 12L. Runway 06 does not intersect Runway 12R but passes behind it. On Runway 06; Ground cleared us to taxi F; S; hold short Runway 12L; switch to Tower. This clearance was partially blocked and Ground repeated it. At the same time; we were running the before takeoff checklist. Holding short of Runway 12L on [taxiway] S; Tower gave us takeoff clearance and we departed without incident.After departure; my Captain re-reviewed the taxi charts and became concerned that we might have crossed Runway 12L; hold short line on Runway 06 prior to receiving the Ground clearance to continue past this hold short line. We were not aware of a hold short line on Runway 06 for Runway 12L. My impression was that we most likely received the clearance to continue at or before this hold short line; but am not absolutely positive. ATC did not mention that we did anything wrong.A note in the company information page alerting pilots to the issue and a hot spot on the airport diagram may be helpful; especially during periods of additive conditions such as frequency congestion; rapid airport reconfiguration of open and closed runways and taxiways; men and equipment operating very near the taxiways and runways. It may help if ATC reinforces runway hold short lines on active runways during this time.