One correction please: SI5 is sent on SACCH not BCCH.
Answer to your question: In normal case i.e. no co-BCCH/BSIC No, he can’t say that “handover failure due to missing definition in BSC” because as we discussed there would be no Handover Command in the first place.
But remember missing neighbor can indirectly cause a Handover failure or call drop. In this case handover may be attempted to a defined neighbor which is not the ‘best’ cell at that location and it may fail due to bad quality or low signal strength. If BCCH of missing neighbor is in Active BA list, you will easily be able to detect it from DT logs.