They are trying to prove abduction. If they can't place the accused in the apartment it doesn't matter how he gained entry, does it?
What if he can be placed in the vicinity of the apartment? What if the accused has a criminal history of breaking and entering ground floor flats in a similar manner?
Let me ask you this: you arrive home one evening to find the sash window in you bedroom pushed up, not how you left it. Nothing in the bedroom appears to be disturbed but then you realise that your great grandad’s gold pocket watch is missing from its place in your bedside table. Do you discount the open window as evidence that the watch was stolen by an intruder because it cannot be corroborated by dna or fingerprints? Must you keep all possibilities on the table, eg you slept walked and put the watch in the bin the night before, or one of your kids played with it, broke it and threw it away, the ghost of your great grandfather spirited it away, all possibilities being equally likely despite the fact the window was not found as you left it?