How did my friend found out I read his facebook messages?

[Jason]

He found out i logged in his facebook and read his messages and now he is really mad. Can someone tell me how he found out? I was very discreet! I only read messages that were already read.

[Tyler]

Uhh, His history maybe?

[Alexisღ]

I don't have a FB so I don't know much about it. Maybe there is a thing that says the time you last logged in and he/she knew they didn't log in at that time, and they just assumed.

[Musa]

When you clicked his message in your inbox then at that time on his side, Facebook showed him in his inbox something like this "last seen <time you read his message>". So, FB has this function that it shows other the time when you read his/her message