Any pro who does commercial work should have a 4-way faucet key in the van. It gives you instant access to the building's fresh water supply even when there's no handle on the faucet. When you need it, this little gadget is worth its weight in gold. Fits most valves and works for 1/4" - 9/32" and 5/16" - 11/32" stems.
4-Way Faucet Key