I'd say that you're using the word "assume" when the correct word is "deduce".respectfully softus, if i don't assume the boiler's firing up the how else is water getting hot?
Fault-finding, in any field, goes like this:
1. Expect.
2. Observe.
3. Theorise.
4. Test the theory.
In other words...
1. Determine how the system should behave.
2. See how it actually behaves.
3. Work out what could account for any mismatch.
4. Do something in order to check that you're correct.
You seem to have skipped step 2, but then you can get help with that here. However, you've rushed to step 3, skimped it a bit, and then omitted step 4 entirely.
But never mind all that. The real question is why something that didn't use to happen is now happening. The first thing to determine is why the one radiator differs from all the others. Once you find that out, the rest should be easy.
For example, with the system idle, and the rogue radiator valves open, do any of the pipes around the pump and zone valves get warm to the touch?