It means that someone (possibly the writer) missed you, that is, someone noticed your absence. Without further context, that’s the extent of my explanation.

You would say "you were missed" to another person in the first person singular when you missed them at an event in the past. You can respond "I missed you too" or "I missed you all too" to respond back.

It means that someone felt your absence and wished that he or she was still with you.

You were probably invited to go somewhere, but did not go. This means that it was a pity you were not there. They would be really happy if you were there = they were missing you there = you were missed. You don’t reply I missed you too. You would reply. I am sorry I could not be there…. I am sorry I was not able to come…. I wish I was there too, but unfortunately…….( Say the reason why…..) Anything along those lines.

It is a passive statement. Say, “thanks, I missed you too.”