No damage that you could see doesn't necessarily equate to no damage. Even if you couldn't see any damage, drivers are required by law to leave their details if the other party wasn't present at the time of the incident. What I assume has happened was the other driver either had a dash camera that was triggered by the bump and recorded your reg plate, or witnesses took down your details. According to the Road Traffic Act, you need to report an accident to the police within 24 hours. You're also supposed to alert your insurance provider as soon as you can, even if you’re not planning to make a claim. Seeing as you didn't do that, you’re potentially guilty of an offence and could end up with a fine and/or points on your licence and/or a charge for criminal damage. I'm afraid we can't predict what will happen in this instance but it's obviously best to comply with police until it's sorted out.