This is a change to plugin behavior. AltDetector 2.03 will now respect the status of vanished players. Previously, if a vanished player joined the server, anyone with the permission altdetector.notify could get an alt detected notice, thereby alerting the recipient that the player joined. The default with this change is to not send messages when a vanished player joins, unless the recipient has the permission altdetector.notify.seevanished.

The same applies when using the /alt command without parameters, because that checks all online players. The permission needed by the command sender in this case is altdetector.alt.seevanished.

Additionally, if you use the plugins SuperVanish or PremiumVanish, AltDetector will use SV's/PV's "Layered See And Use Permissions", which creates a hierarchy of vanished players, and will only show messages about the players visible to the message recipient.

The new AltDetector seevanished permissions default to op. If you want someone to see AltDetector messages about vanished players, they will need one or both of the new permissions. To retain AltDetector's current behavior, simply add the ...notify.seevanished subpermission to anyone with the ...notify permission.

Thanks to TreemanKing for the suggestion.