If you want to be truly blameless in terms of metagaming, RP the uncertainty. Maybe even take a turn for the "what just happened!?!" factor.
A relevant modern analogy would be the difference between knowing someone has a particular disease versus knowing the list of symptoms. If you know someone has a flu virus, you know basically what's going to work and what's not. If you know that someone has a mild fever, headache, and congestion, you have some general ideas of what is going to help in that "bed rest and fluid" sort of theme, but it could be anything from allergies to pneumonia with a lot of stops in between and a lot of different sorts of treatments.
If you knew it was invisibility, you would know he was still in the cell. Since you don't know it was invisibility, you don't know whether he could still be in the cell, but it's in the range of possibilities, among many others. Checking the cell to make sure that it's not an illusion or other trickery seems pretty obvious (and, frankly, with a DM worth his or her salt, will still allow the mage a jump on things, giving 'game value' to the invisibility and reducing the depth of a question of metagaming).
He either teleported or went invisible. If he went invisible then doing the blind mans bluff act around the room might find him. If he didn't, then you may look like an idiot but there's no one there to see it.
So no, i don't think its metagaming to act as if he went invisible.
I disagree on one point. Posters have been saying that since he cast it when you opened the door, it's less likely to be a teleport than invisibility. However, the door was opened to allow a barbarian to go in and beat the #$#&* out of him. If I was a wizard with a choice, that's when I would teleport.
However, I do agree that it would not be metagaming to wave your arms around, maybe throw some flour or dirt into the air, to see if he's still there. Since that would be the normal reaction if someone disappeared in your kitchen, in a world without invisibility. It would seem quite likely in a world with magic. Note - 8 int means he might leave the door open until it's too late, though.
I don't think so. Wisdom covers situational awareness. The situation is that the door is wide open. Even wild animals know how to cut other animals off. In this case the cut off point is the door.
