I was battling the wolves and one of them relieved me of my arm, causing me to drop sall thuul. when it happend i got the message about my arm and the weapon dissapearing (and now I cant find it). Is it gone? what happened?

Somebody probably stole it. They don't just "disappear"
If you have detect material try searching for "mithril", that might work. I'm not sure if that works on equipped items though.

So now I have an invisible enemy running around on the werewolf level wielding salthul? YAY! will infravision help me see invisible?

Depends on who's wielding it. It's also possible that they just picked it up and are carrying it (like a giant magpie or something)

If the invisible guy is Intelligent, then you can see him through ESP. If he is warm-blooded, you can see him through Infravision. That's why the spider silk golem is so dangerous - invisible, no warm blood and not intelligent.

Most likely there was an eddy in the space time continuum. they teleport items away giving the message"the XXXXX on the ground disappears"

Ah, good point slob.