That's Davel's theory about Holly's theory, isn't it? Holly's theory includes someone watching and noticing that the children were being left alone and that the patio doors were being left unlocked every evening. The lady you mention wasn't there to do that imo.
My theory is that the abductor was someone who knew MM was 'home alone' in the unsecured apartment and what the McCanns MO was of an evening. This knowledge could have been gained first hand by observation or second-hand from someone else who was in a position to gain that information.
PF claims she heard MM crying on 1st May calling out 'Daddy, daddy' from 10.30 pm - 11.45 pm which stopped when she heard the patio doors slide open which she assumed was the McCanns returning. PF claims she called a local friend to discuss the situation. Who else, if anyone, did PF tell and was it passed on to anyone else by PF's local friend, Edna Glyn? Effectively they were letting others know MM was home alone in the unsecured apartment.
By all accounts the nights were cold and dark so I can't see too many people spending enough time on balconies to observe the McCanns MO directly. I think the abductor(s) gained the knowledge from PF directly or indirectly.