Strange Things have been happening in my world?

[Tameable]

So me and my buddy are playing on 1.21.0-rc6, and some stuff that is too strange to be bugs have been happening.
 

so what has happened is that awhile back when my friend was traveling back I could see someone in the distance, in the northwest near our house. my friend was still 10000 blocks out from our base (we are playing in homo sapien mode) and I get some weird feeling about it.

Late and more recently, my friend saw someone else again off by a pillar we made to help find our ways back, while I was building our new house.
then the big one happened where our entire forest caught on fire. there where no fires or anything that could of started this, and it was huge. dead animals across the area and my friend found a lone granite cobblestone block out in the fire area. we've also taken random damage out of nowhere (this could be a bug tho)  

if anyone knows whats happening please let me know, the server is a whitelist only and nobody else has seem to joined except my friend. it's scaring both of us.
(Images attached from the fires, as well the damage of the fires on the map)

[CastIronFabric]

are you running a public server without a password?

[Thorfinn]

Fires can be lightning, assuming that's enabled, but @CastIronFabric is spot on. Regardless of the answer, create/change your password.

You need to give more information if you want more help. Mods, at the very least.

[LadyWYT]

By "someone else", did you/your friend actually see a nametag? If there was no nametag then it was likely a bear standing on its hind legs, or something else that looks similar to a player but isn't(if this is a modded server, make sure you aren't running a mod like Outlaws that adds other humanoids). If it was just a marker on the map, it was probably a desync issue, as that's happened on my friend's server a time or two. Same as for actually spotting a player tag--if it's not moving, it's probably a ghost of one of you two caused by a desync; I think that issue was supposed to be fixed, but it could have cropped back up. I know that's happened on friend's server too.

That being said...the server logs should tell you for sure whether or not you had another player log in. As others have mentioned, I'd check your server settings as well to make sure that you aren't running a public server by accident. That happened when first setting up a server with my friend--the server was private, but settings were messed up so it was listed publicly and a random player joined. Thankfully, said random was polite enough to leave without issue when asked, and a whitelist has prevented that issue from reoccurring. So...yeah, whitelist and/or server password to keep out unwanted guests.

As for the forest fire, you probably just have the setting enabled that allows lightning to start fires. This setting is only enabled by default on the Wilderness Survival difficulty. If it's not a desirable effect, I would check your server settings and see about setting that particular game rule to "false" to prevent it from happening again.