Tell me: is the park open after dark, or do they shut it at sunset? If it's open, then you might actually have a regular pattern of locals going through it to get to and from their homes. Along with familiarity, this might have created a false sense of security to Juliana. However if the park is locked after dark then one needs to think about how she was found there. Is it easy to get into?
The Haringey Council information page about the park (http://www.haringey.gov.uk/librarie...-spaces/z-parks-and-open-spaces/finsbury-park) is vague on this point. It says: 'Opening and closing times of the park are dependent on season, but generally the park is open at dawn and locked when it gets dark.'
Unlike many municipal parks that are enclosed by metal railings, Finsbury Park is surrounded by fencing made of wooden slats or planks. Here and there the fence decays or is deliberately damaged, so even if the gates were locked it wouldn't be that hard to get in if you knew where there were gaps in the fence.