Stalker,
Undefined,
17 years ago
It's a bit of movie magic there I'm guessing, as things like trophy skulls that the predator might have on it are also cloaked, despite the fact that they are not a part of its armour.
A possibly theory is that along with the main cloaking field generator on the predator's left wrist gauntlet, there may be ways for it to implant small recievers or even secondary generators inside external items on its body, such as hanging skulls, etc. which will activate along with the main cloak, & camouflage loose items. This would fit, since prey wouldn't have any such device on their bodies, & therefore wouldn't be affected by the cloak.
As far as the weapons uncloaking at certain points, it is quite possible that they are able to be individually switched between visible & camouflaged.