Observing the apartment in the daytime to what end? Most of the time the occupants were out, so not looking for a 'window of opportunity'. The quiet dark evenings were the time to observe the checkers and work out where the gaps were. No-one would have seen them either it because it was so deserted, apparently.
In a pre-planned abduction scenario, it makes sense to do daytime checks to use the light to establish the layout of the intended crime scene.
If it turns out that the unlocked patio doors were the point of entry, then one needs to tie up - children left in 5A (or block 5) - parents not in 5A (thus opportunity) - and knowledge of parents leaving the patio door unlocked.
The latter comes about with a night time check only, as this group of events only happened at night time, and not during the day.
It is also the case that abducting during daylight is significantly riskier than abducting when things became quiet at night. Thus suggesting a good squiz in the dark, checking on lighting and car and pedestrian traffic.
Day checks for layout. Evening checks for tying things together and/or operational checks.
