Netplay weirdness, was I hacked or was it just a random thing?

[ElStego]

I recently setup some Pies (3B+ running 4.5) for me and a few friends with a custom image I made. We setup Netplay with WIFI without a hitch and have played several times over the last few weeks without issue. Usually my friend hosts, and sometimes I will. We were playing late last night and had an odd glitch that made me wonder if maybe we were hacked (I will preface this by saying I didn't realize I should change my raspberry pie system user password for security purposes, and I have since fixed this).

We were playing a NES game that always works and my friend was hosting. He had been having internet connection issues as we played and was disconnected several times throughout the night. When the error happened he had just had a connection glitch several minutes before, but he was able to rejoin quickly without much effort. After several minutes he informs me that he saw an odd error message that he couldn't quite read at the bottom of his screen (it did not appear on mine) in the retro-arch or system yellow font (I don't know what its formal terminology would be but its the stuff that always comes up while playing). He said it was much longer than usual spanning almost the entire length of the bottom of the screen, and what he could make out had the words "Warning" and "internet" in it. I would have certainly chalked this up to his connection issues, but a few seconds later both of us saw a "Anonymous has joined as Player 4" message. There were only three of us playing and my other friend was in the process of disconnecting, so it very well could be a combo of him disconnecting and my other friend's internet issues. That being said, the anonymous player joining message seemed odd to me. We all are the only people we play with and we are the only ones we have given our external IP addresses to in order to play. While I now know to change the pi passwords for safety, I was under the impression that no one could access these games in the first place if they don't have our external IPs, which is why I am inclined to believe it was just a few connectivity error messages which we weren't able to read in full. What do you guys think? Do I need to worry about having been hacked or am I being overly worried about that? I already did my due diligence with my home network and nothing seems out of the ordinary. I don't know how to check the pi for malware or evidence of external hacking so I am also asking so I know if I should wipe and reflash our systems or if I am good just making sure we all change our pie passwords so we are protected next time we play. Thanks so much for the input everyone, really appreciate it.

[mitu]

I don't think you've been hacked, it's either a connection glitch or somebody joined by mistake (?). Changing your Pi's default password it's always a good idea and it's one of the first steps that should be done on a new install.

[BGallagherLA]

I've been playing various Netplay games with my brother who lives in another state, and whenever we play a 4-player game like TMNT or Simpsons arcade, we almost always get some random "Anonymous" person who joins and plays for a bit. I know you can have password protected games on Netplay but I'm not sure how to set that up.

[ElStego]

@mitu thanks for the reply. changed the password right after. I'm thinking it must have been a connection glitch because it seems incredibly unlikely that someone would have been playing an obscure NES game with the exact same rom, emulator, and Retropie version at 1am EST while also happening to accidentally use our External IP.

[ElStego]

@BGallagherLA wondering if that is just an automatic thing with NetPlay and is just a computer player or something rather than someone actually joining? I just don't understand how someone can randomly join a game when you need the proper external IP to be able to access the game, and the exact same rom set, emulator and RetroPie version in order for it to work