Hot water won't stop coming out.

We just started a restaurant expansion.

Plumber went in there and is installing an AO Smith Cyclone in the expanded area and replacing a Rinnai tankless.

When he shut off the cold water supply at the Rinnai and went to a sink to relieve the pressure the water never stopped coming out. The pressure dropped to a slow trickle but never stopped coming out.

He then shut off the main water supply to the suite and the water stopped immediately.

He mentioned there may be a cross connection but didn't have the time to troubleshoot it at the moment and would be doing that in a day or two.

Are there areas where a plumber can make a simple mistake and connect a cold supply to a hot supply?

Only fixtures that I can think of that have mixing valves are in the restrooms. I can isolate these fixtures by shutting off the angle stops but if that's not the problem, any advice on how to check for a cross connection?

I think you are on the right track. There is probably mixing valves at the hand sinks and perhaps a tempering valve at the water heater. Turn off the stops and see if that does the trick.

Because the outlet temperature of a typical tankless can vary a huge amount based on the volume drawn, and the incoming water temperature, it is not at all unusual to plumb it with a tempering valve to limit the potential high water temperature. It should be fairly easy to spot, as it would be usually right where the tank was removed. But, any tempering valve in the system could feed back some water.

Don't leave out the possibility that the cold inlet valve is leaking a bit.

What style are the restroom faucets?
If they are the automatic ones with the electric eye and a small mixing valve under the cabinet, this is likely where the problem lies...
the little mixing valves commonly leak back causing cross connections...

Pre-rinse sprayer? Mop faucet used as feed for chemical/soap dispenser?

The restroom faucets have automatic faucets.

Would it be best to shut off all accessible angle stops, shut off the cold at the Rinnai, and open a faucet (mop sink).

If it stopped, go back and turn them on 1 by one?

If it didn't stop, then what?