Well, just like any other dream sign, something out of the ordinary happening is a good trigger to cause lucidity. For example, I was being chased by cops once, and realized how out of the normal that was for me, and realized I was dreaming. So yes, missing an object you just had would make a good trigger. Also, the fact it's a repeated dream gives and even better chance for becoming lucid, because your brain becomes more and more aware it's dreaming when that chain of events happens.

Good luck with using it as a trigger!