Empty robins' nest

I had a pair of robins nesting in a box in our garden. There were at least 4 eggs which were then sat on  for a couple of weeks. A few days ago the birds' behaviour changed and they were going to and fro from the nest, I assumed that the eggs had hatched.

Today there was no sign of any robins. The nest is empty, with no sign of chicks (dead or alive) or egg shells.

Have I scared them into abandoning their nest by showing too much interest, or is it likely they've been predated?