The case of the dissapearing bulbs

Hannah dug this area in our front lawn a few months ago and planted some iris bulb (she had an excess of iris bulbs) then one day they were gone!  So we replaced them with more bulbs that Hannah had left in a different part of the yard and a couple days later.... gone again.  It can't be an animal because they were so meticulous picked out to get every last one without leaving a trace.  But who is up to this?  Our neighbors?  Some kids? The city?  Our landlord?
We are seriously stumped and a little freaked out.