There was a cobweb at the bottom left corner of that window (looking from inside).
But lets look at the window logically shall we? John tells police when they first arrived that he had gone through the house looking specifically for a point of entry. He even goes to the garage to check the door that leads to the house. He does not mention the opened basement window. He later does another tour of the basement and does not mention the opened window, nor does Fleet White who went down before him. None of the police offers that examine the basement notice it being opened either. In fact it is not until four months later that John reveals that he had noticed the window was opened and that he had shut it without telling anybody.
So lets go a step further. There is also a damaged door on the main level that the Ramsey's claim was new. Friends would later state that that damage had been there previous that that Patsy was aware of it. So we already have the parents lying about the possible upstairs entry point, so what do you think the odds are that John's ridiculous story is actually true?
Lastly, there is a chair on the outside of the train room door that John says he had to move to gain entry. If an intruder needs something to stand on to get out that window, wouldn't a chair be far better than a suitcase? The intruder would have had to close the train room door whilst pulling that chair in front of it. Why?
But the fact that John tells cops that he checked all doors and windows as soon as the cops arrived, then later says he probably didn't check any of them is cause enough to say WTF. And they wonder why the cops considered them as suspects from the outset???