If I offer to investigate he says theres probably nothing to worry about.

I've tried going there anyway but w/o the quest the attack on Ranger Station Charlie hasn't occured.

I haven't started Return to Sender so thats not causing the problem.

This is bugging the completionist in me.

Strange, when you end the conversation, he should ask you to check anyway, no matter which dialogue option you choose. The wiki entries on Ranger Andy and the quest Andy and Charlie don't list this as a bug either. Are you on PC - ie are you running any mods that could cause this or could you use console commands to fix it?

I'm on pc but the only mods I'm running atm just add extra hairstyles and eye colours. Never had any problems getting this quest before.