If the relief is leaking be sure you are not at 30 psi or more in the loop

The fast fill below is often the culprit. It keeps the loop at 12 psi (cold) unless you are heating a 3rd floor on the loop. When the water heats up the pressure will rise to 22-25 psi. If you are tickling 30 that is why it is weeping.

When the fast fill begins to fail water seeps by. Street pressure is 90 psi. The loop creeps up over 30, lifts the relief and is ok for a day or 2.

Opening the schrader just releases the air charge in the bladder. If water did not come out, it is not likely the expansion tank.
Close the feed valve like grumpy said and your problem will go away for now.
If you find that there is gurgling in the pipes you will have to add water manually.

Take Grumpys advice and wait until spring and you no longer need heat.
It doesnt show where you live but it is likely you will need heat.
If you go ahead with this job, it will likely be at night or on a weekend. If you need heat, and the job goes wrong, you will have to call out a qualified technician and pay overtime for after hours emergency service.
If you wait until spring, you have all summer to put parts in and figure it out.

The relief on that loop is not a T&P.
It is just a pressure relief that lifts if the loop pressure exceeds 30 psi.

So...?

He changed the fast fill valve.
See post #3.

I suspect that if it is not leaking now, it was just overfilled and the water expanded beyond what the expasion tank could handle.
You will know over the next few days.

It is too bad the oil tech did not tell you he didnt work on gas boilers.
A tankless boiler is way more complicated than an oil boiler.