If you turn off the water to the auto-fill valve to the boiler, and the pressure continues to rise over a few days, that water has to be coming from somewhere, eh? The only likely suspect would be the potable water coil in the indirect, since it's still at the higher pressure. If it doesn't HAVE an auto-fill valve, the experiment is already been done.
Sometimes auto-fill valves leak-through if the valve seats are pitted or get grit in them, which could be another reason for the pressure to rise over time. But if you've turned off the pressure to the potable side of the valve, the heating system's pressure won't continue to rise. As a matter of course I always turn off the feed to the auto-fill after air-purging and pressurizing the system. In the event of a system leak the pressure could then fall, but the boiler makes enough sizzle pop & bang when running at low pressure that it would be noticed well before it became a safety issue. That way the water volume of the leak is also bounded, should a leak occur.
The heating system water is never going to get into the potable through that coil until the pressure on the heating system exceeds that of your potable supply. Since most heating systems are running at well under 20psi, and most potable is running 30psi+, that's just not going to happen if you control the heating system's pressure. Some heating system components aren't designed to operate at potable-type pressures, and you really can't/shouldn't let it get very high. Most are set up in the ~12psi range (static, no pumping), which is plenty for most boilers in 1 & 2 story houses.