I don't actually think Xenon-124 being radioactive was a surprise. All the publicity related to observing the decay for the first time is phrased around 'hey we observed something that's very rare' not 'hey this thing happened we didn't expect to happen'.
Which seems to point even more towards, scientists have a pretty good handle on which ones are radioactive.
Which seems to point even more towards, scientists have a pretty good handle on which ones are radioactive.