How to figure out where this is coming from? Minecraft related?

I'm a manager on a Minecraft server and can't seem to figure out why, out of the blue, I (and other members) are given SPEED and NIGHT VISION at random times. Here is the log info to show exactly what happens:

[Server thread/INFO]: @: Given SPEED (ID 1) * 10 to IGN HIDDEN for 600 seconds
[Server thread/INFO]: @: Given NIGHT_VISION (ID 16) * 0 to IGN HIDDEN for 600 seconds

I removed the IGN for privacy purposes.
This happens throughout the map so I'm sure it's not a command block. If it was a command block it would have show it - for example:

[Server thread/INFO]: CommandBlock at CORDS issued server command: /tp IGN CORDS

I can't figure this out. I'm sure it will require an xpert answer.

I do have access to files on the server and have been searching for a resolution and have been unsuccessful.

This just started happening a week ago.

Guest

Someone must have been hacking to your server. I would kick the guy/girl first, and if he still does that 1 more time. After that I will ban him. So it's 2 times kick, and 1 time ban.

Alexander The

Has it happened to anyone else or is it just that one person? If it is that one person then it is highly likely that he is able to give himself items through the server console. Have you set up any anti-cheat plugins?

Ducky

Mostly a plugin issue if there are new plugins you have that are in beta those are worth getting things looked into and if you don't fix it act it off as an attraction to the server like random effects every minute so come join now!

Jake