If it was an artifact - especially a discarded solar sail, explaining the acceleration - there might be a follow-up.

Imagine you were making a sail-propelled probe, with the intent of slowing the probe so it was captured by the target system. So the probe would have two sails. The first would be deployed after launch, to catch the initial laser beam, then later solar winds, to accelerate to system escape velocity.

You'd then detach that sail, and cruise. Later, you'd deploy a second sail to start braking on your way into the target system, to slow to capture velocity.

The first thing the inhabitants of the target system would see would be the first, discarded, sail as it tumbled through their system and accelerated back out ...