Can a player use the Wand of Dowsing card during a combat which is not his?
This should help you.

Q. The Wand of Dowsing doesn't say &quot;Usable once only,&quot; but I have to discard it after I use it. Can I treat it like a one-shot (play it from my hand rather than the table, or play it during combat)?
A. The clear intent is that this is a one-shot card, even though it lacks the magical &quot;Usable once only&quot; language, and so the rules of one-shot cards apply to this one as well. It can be played during combat, and it can be used from the hand. We've fixed this error in recent printings.

Source: Munchkin FAQ